diff --git a/AGENTS.md b/AGENTS.md
index bfbbce5..ee5051e 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -9,7 +9,7 @@ Canonical Go client for OnlyOffice Workspace (Projects + Calendar + CRM) and the
   - `request.go` — `Request`, `Query`, `Time`, `Token`, `MetaResponse`, `Permissions`.
   - `auth.go` — `Authenticate`, `AuthenticateContext`, `InvalidateToken`, `Auth`, token lifecycle.
   - `http.go` — transport + DRY response decoders (`ResponseArray`/`ResponseObject`/`postFormObject`/`putFormObject`/`deleteObject`).
-  - `projects.go`, `tasks.go`, `users.go`, `calendar.go`, `crm.go`, `files.go`, `mails.go` — typed / untyped domain methods. **`files.go`** — CRM opportunity upload plus **project/task Documents** (`GetProjectFiles`, `UploadProjectFile`, `GetTaskFiles`, `AttachFilesToTask`, `UploadTaskFile`, `DetachTaskFile`, `GetFile`, `RenameFile`, `DeleteFiles`, `DownloadFile`). **`mails.go`** — OnlyOffice Workspace Mail addon (`ListMailAccounts`, `ListMailFolders`, `ListMailMessages`, `GetMailMessage`, `RemoveMailMessages`).
+  - `projects.go`, `tasks.go`, `users.go`, `calendar.go`, `crm.go`, `files.go`, `mails.go`, `invoices.go` — typed / untyped domain methods. **`files.go`** — CRM opportunity upload plus **project/task Documents**. **`mails.go`** — OnlyOffice Workspace Mail. **`invoices.go`** — CRM invoices, PDF regen/cleanup, status. Association rules: [`docs/crm-associations.md`](docs/crm-associations.md).
   - Pure stdlib + `google/go-querystring`; no UI, no dotenv.
 - **CLI — `cmd/oo/` as `package main`.** Cobra wrapper that loads `.env` via `godotenv` at startup. **Subject-based command tree** mirroring [`tea`](https://gitea.com/gitea/tea):
   - `main.go` — entry point (docstring lists the command tree).
diff --git a/CHANGELOG.md b/CHANGELOG.md
index a399d00..6d83a24 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,18 @@ All notable changes to this project are documented here. The format is based on
 [Keep a Changelog](https://keepachangelog.com/en/1.1.0/), and this project
 adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Added
+
+* **docs:** CRM association graph and OO quirks (`docs/crm-associations.md`)
+* **crm:** `ForceRegenerateInvoicePDF`, `SetInvoiceStatus`, `PurgeStaleInvoicePDFs`, contact/opportunity file list helpers
+* **oo:** `invoices pdf`, `pdf-cleanup`, `status`; create `--consignee`; draft-invoice force-regens PDF
+
+### Fixed
+
+* Document that invoice→deal must be set at create (`update --opportunity` often HTTP 400)
+
 ## [0.9.0](https://github.com/eSlider/go-onlyoffice/compare/v0.8.3...v0.9.0) (2026-07-26)
 
 
diff --git a/README.md b/README.md
index 78ff066..bcfd1ae 100644
--- a/README.md
+++ b/README.md
@@ -623,15 +623,13 @@ oo tasks files detach 208 12345
 | `projects` | `list`, `get`, `milestones`, `create`, `update`, `delete`, **`files`** (`list`, `upload`, `download`, `rename`, `delete`) |
 | `tasks` | `list`, `get`, `create`, `update`, `delete`, `subtask add`, **`files`** (`list`, `upload`, `detach`) |
 | `users` | `list`, `self` (alias: `oo whoami`) |
-| `contacts` | `list`, `get`, `delete`, `info-add` |
-| `persons` | `list` (filtered), `create`, `delete` |
-| `companies` | `list` (filtered), `create`, `delete` |
-| `contacts` | `list`, `get`, `delete`, `info-add`, `dedupe-info` |
+| `contacts` | `list`, `get`, `delete`, `info-add`, `merge`, `dedupe-info` |
 | `persons` | `list`, `create`, `delete`, `dedupe` |
 | `companies` | `list`, `create`, `delete`, `dedupe`, `dedupe-persons` |
 | `opportunities` | `list`, `get`, `create`, `delete`, `stages`, `member-add`, `dedupe`, `dedupe-members`, `fix-titles` |
+| `invoices` | `list`, `get`, `create`, `update`, `pdf`, `pdf-cleanup`, `status`, `delete`, `items …` |
 | `crm` | `cleanup` |
-| `mails` | `accounts`, `folders`, `list`, `get`, `delete` |
+| `mails` | `accounts`, `folders`, `list`, `get`, `draft`, `attach`, `draft-invoice`, `delete` |
 | `cases` | `list`, `create`, `delete`, `member-add` |
 | `crm-tasks` | `list`, `create`, `delete`, `categories` |
 | `applications` | `sync` |
@@ -718,6 +716,32 @@ oo opportunities dedupe-members
 oo opportunities fix-titles
 ```
 
+Merge two known company ids (keeps `INTO`):
+
+```bash
+oo contacts merge FROM_ID INTO_ID
+```
+
+Company ↔ person ↔ deal ↔ project ↔ invoice ↔ mail rules and OO quirks:
+[docs/crm-associations.md](docs/crm-associations.md).
+
+### Invoices (`oo invoices`)
+
+**Problem:** Bill a client deal as Draft, regenerate PDF, prune duplicate PDF
+attachments, prepare OnlyOffice Mail — without inventing a second bill-to company.
+
+```bash
+# Always pass --opportunity at create (update --opportunity often HTTP 400)
+oo invoices create --number INV-2026-01 --contact COMPANY_ID --item ITEM_ID \
+  --price 300 --opportunity DEAL_ID --language de-DE \
+  --line-description "…" --terms $'…' --description $'…' --po "Deal #DEAL_ID"
+
+oo invoices pdf 41 --force
+oo invoices pdf-cleanup INVOICE_ID
+oo invoices status INVOICE_ID --status draft
+oo mails draft-invoice --invoice INVOICE_ID --to info@client.de
+```
+
 **Deal grouping flag** — when the same role at the same company created
 separate deals (`Engineer @ Acme` vs `Engineer`):
 
@@ -821,8 +845,21 @@ oo mails get 5664 -o json | jq '{subject, from, to, date}'
 # Remove one or more messages (server moves to trash or deletes per Mail rules)
 oo mails delete 5664
 oo mails delete 5664 5663 5661
+
+# Create / update a draft (HTML body field is API "body"; plain text is wrapped)
+oo mails draft --to client@example.com --subject "Rechnung INV-2026-01" \
+  --body "Guten Tag,\n\nanbei die Rechnung.\n\nMit freundlichen Grüßen"
+
+# Attach an OnlyOffice Files document (e.g. invoice PDF file id) to a draft
+oo mails attach 7301 --file-id 12345
+
+# Regenerate invoice PDF (force) + draft + attach (does not send)
+oo mails draft-invoice --invoice 16 --to info@example.com
 ```
 
+Matrix / chat URLs in signatures: use plain text
+`chat: https://matrix.to/#/@user:server` — HTML `<a href="…#…">` truncates at `#`.
+
 **Table output** splits the `from` header into `fromName` and `fromAddress`
 (e.g. `Bitfinex` + `no-reply@bitfinex.com`). **JSON output** returns the raw
 API payload.
diff --git a/cmd/oo/cli_test.go b/cmd/oo/cli_test.go
index 9f5eca9..66467dc 100644
--- a/cmd/oo/cli_test.go
+++ b/cmd/oo/cli_test.go
@@ -11,7 +11,7 @@ func TestRootRegistersSubjects(t *testing.T) {
 	want := []string{
 		"calendar", "projects", "tasks", "users", "whoami",
 		"contacts", "persons", "companies",
-		"opportunities", "cases", "crm-tasks", "applications", "crm", "mails", "catalog",
+		"opportunities", "cases", "crm-tasks", "applications", "crm", "mails", "catalog", "invoices",
 	}
 	got := make(map[string]bool, len(rootCmd.Commands()))
 	for _, c := range rootCmd.Commands() {
@@ -38,7 +38,7 @@ func TestRootHelpListsSubjects(t *testing.T) {
 		t.Fatal(err)
 	}
 	help := out.String()
-	for _, snippet := range []string{"calendar", "projects", "tasks", "users", "opportunities"} {
+	for _, snippet := range []string{"calendar", "projects", "tasks", "users", "opportunities", "invoices"} {
 		if !strings.Contains(help, snippet) {
 			t.Fatalf("help missing %q", snippet)
 		}
diff --git a/cmd/oo/common.go b/cmd/oo/common.go
index 5f5ab27..b9b3592 100644
--- a/cmd/oo/common.go
+++ b/cmd/oo/common.go
@@ -20,7 +20,7 @@ var outputFormat = "table"
 var rootCmd = &cobra.Command{
 	Use:           "oo",
 	Short:         "OnlyOffice Workspace CLI — subject-based command tree",
-	Long:          "oo is a thin CLI over github.com/eslider/go-onlyoffice.\nCommands are grouped by OnlyOffice subject (calendar, projects, tasks, users, persons, companies, opportunities, cases, crm-tasks, applications, catalog).",
+	Long:          "oo is a thin CLI over github.com/eslider/go-onlyoffice.\nCommands are grouped by OnlyOffice subject (calendar, projects, tasks, users, persons, companies, opportunities, cases, crm-tasks, applications, catalog, invoices).",
 	Version:       version,
 	SilenceUsage:  true,
 	SilenceErrors: false,
diff --git a/cmd/oo/contacts.go b/cmd/oo/contacts.go
index 5d838c6..aa0f533 100644
--- a/cmd/oo/contacts.go
+++ b/cmd/oo/contacts.go
@@ -201,7 +201,7 @@ func contactsInfoAddCmd() *cobra.Command {
 }
 
 func personsCreateCmd() *cobra.Command {
-	var first, last, email, linkedin string
+	var first, last, email, linkedin, phone string
 	var companyID int
 	var jobTitle, about string
 	cmd := &cobra.Command{
@@ -224,6 +224,9 @@ func personsCreateCmd() *cobra.Command {
 			if email != "" {
 				_, _ = c.AddContactInfo(cmd.Context(), pid, "Email", email, "Work", true)
 			}
+			if phone != "" {
+				_, _ = c.AddContactInfo(cmd.Context(), pid, "Phone", phone, "Work", true)
+			}
 			if linkedin != "" {
 				_, _ = c.AddContactInfo(cmd.Context(), pid, "LinkedIn", linkedin, "Work", false)
 			}
@@ -237,6 +240,7 @@ func personsCreateCmd() *cobra.Command {
 	cmd.Flags().StringVar(&jobTitle, "job-title", "", "")
 	cmd.Flags().StringVar(&about, "about", "", "about / bio")
 	cmd.Flags().StringVar(&email, "email", "", "primary email (adds ContactInfo)")
+	cmd.Flags().StringVar(&phone, "phone", "", "primary phone (adds ContactInfo)")
 	cmd.Flags().StringVar(&linkedin, "linkedin", "", "linkedin url (adds ContactInfo)")
 	return cmd
 }
@@ -347,7 +351,7 @@ func contactEmailsFromMap(p map[string]any) []string {
 }
 
 func companiesCreateCmd() *cobra.Command {
-	var name, email, website string
+	var name, email, website, phone, about, street, city, state, zip, country string
 	cmd := &cobra.Command{
 		Use:     "create",
 		Aliases: []string{"add"},
@@ -365,12 +369,34 @@ func companiesCreateCmd() *cobra.Command {
 				return err
 			}
 			cid := strconv.Itoa(int(flexIDFloat(out["id"])))
+			if about != "" || street != "" {
+				aboutText := about
+				if street != "" {
+					addr := strings.TrimSpace(strings.Join([]string{street, zip, city, state, country}, ", "))
+					if aboutText != "" {
+						aboutText = aboutText + "\n" + addr
+					} else {
+						aboutText = "Billing address: " + addr
+					}
+				}
+				if updated, err := c.UpdateCompany(cmd.Context(), cid, name, aboutText); err == nil {
+					out = updated
+				}
+			}
 			if email != "" {
 				_, _ = c.AddContactInfo(cmd.Context(), cid, "Email", email, "Work", true)
 			}
 			if website != "" {
 				_, _ = c.AddContactInfo(cmd.Context(), cid, "Website", website, "Work", false)
 			}
+			if phone != "" {
+				_, _ = c.AddContactInfo(cmd.Context(), cid, "Phone", phone, "Work", true)
+			}
+			if street != "" {
+				if _, err := c.AddContactAddress(cmd.Context(), cid, street, city, state, zip, country, "Billing", true); err != nil {
+					fmt.Fprintf(cmd.ErrOrStderr(), "warning: address API failed (stored in about): %v\n", err)
+				}
+			}
 			printObject(out)
 			return nil
 		},
@@ -378,6 +404,13 @@ func companiesCreateCmd() *cobra.Command {
 	cmd.Flags().StringVar(&name, "name", "", "company name")
 	cmd.Flags().StringVar(&email, "email", "", "primary email (adds ContactInfo)")
 	cmd.Flags().StringVar(&website, "website", "", "website url (adds ContactInfo)")
+	cmd.Flags().StringVar(&phone, "phone", "", "primary phone (adds ContactInfo)")
+	cmd.Flags().StringVar(&about, "about", "", "about / notes")
+	cmd.Flags().StringVar(&street, "street", "", "billing street")
+	cmd.Flags().StringVar(&city, "city", "", "billing city")
+	cmd.Flags().StringVar(&state, "state", "", "billing state")
+	cmd.Flags().StringVar(&zip, "zip", "", "billing zip")
+	cmd.Flags().StringVar(&country, "country", "", "billing country")
 	return cmd
 }
 
diff --git a/cmd/oo/invoices.go b/cmd/oo/invoices.go
new file mode 100644
index 0000000..da6555c
--- /dev/null
+++ b/cmd/oo/invoices.go
@@ -0,0 +1,441 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+	"time"
+
+	onlyoffice "github.com/eslider/go-onlyoffice"
+	"github.com/spf13/cobra"
+)
+
+var invoicesCmd = &cobra.Command{
+	Use:     "invoices",
+	Aliases: []string{"invoice"},
+	Short:   "CRM invoices (billing)",
+}
+
+func init() {
+	rootCmd.AddCommand(invoicesCmd)
+	invoicesCmd.AddCommand(invoiceListCmd())
+	invoicesCmd.AddCommand(invoiceGetCmd())
+	invoicesCmd.AddCommand(invoiceCreateCmd())
+	invoicesCmd.AddCommand(invoiceUpdateCmd())
+	invoicesCmd.AddCommand(invoicePDFCmd())
+	invoicesCmd.AddCommand(invoicePDFCleanupCmd())
+	invoicesCmd.AddCommand(invoiceStatusCmd())
+	invoicesCmd.AddCommand(invoiceDeleteCmd())
+	invoicesCmd.AddCommand(invoiceItemsCmd())
+}
+
+func invoiceListCmd() *cobra.Command {
+	var count, offset int
+	cmd := &cobra.Command{
+		Use:   "list",
+		Short: "List CRM invoices",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			list, total, err := c.ListInvoices(cmd.Context(), count, offset)
+			if err != nil {
+				return err
+			}
+			rows := make([]map[string]any, 0, len(list))
+			for _, inv := range list {
+				rows = append(rows, onlyoffice.FlattenInvoiceRow(inv))
+			}
+			if outputFormat == "table" {
+				fmt.Printf("total: %d (shown: %d)\n", total, len(rows))
+			}
+			printTable([]string{"id", "number", "cost", "statusTitle", "contactName", "issueDate", "dueDate"}, rows)
+			return nil
+		},
+	}
+	cmd.Flags().IntVar(&count, "count", 50, "")
+	cmd.Flags().IntVar(&offset, "offset", 0, "")
+	return cmd
+}
+
+func invoiceGetCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "get INVOICE_ID",
+		Short: "Show an invoice by id (incl. lines)",
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			out, err := c.GetInvoice(cmd.Context(), args[0])
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+}
+
+func invoiceCreateCmd() *cobra.Command {
+	var (
+		number, issueDate, dueDate, language, currency, terms, description, po string
+		contactID, consigneeID, itemID, opportunityID                          int64
+		price, qty                                                             float64
+		lineDesc                                                               string
+	)
+	cmd := &cobra.Command{
+		Use:   "create",
+		Short: "Create a draft invoice with one line",
+		Long: `Create a CRM invoice (Draft) with a single line.
+
+Always pass --opportunity when a deal exists (entity link at create). Updating
+--opportunity later often fails with HTTP 400 — see docs/crm-associations.md.
+
+Example:
+  oo invoices create --number INV-2026-01 --contact CONTACT_ID --item 12 \
+    --price 300 --opportunity OPPORTUNITY_ID --line-description "Service package" \
+    --terms "…payment terms…"
+`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if number == "" || contactID == 0 || itemID == 0 {
+				return fmt.Errorf("--number, --contact and --item are required")
+			}
+			if issueDate == "" {
+				issueDate = time.Now().Format("2006-01-02") + "T00:00:00.0000000+01:00"
+			} else if !strings.Contains(issueDate, "T") {
+				issueDate = issueDate + "T00:00:00.0000000+01:00"
+			}
+			if dueDate == "" {
+				dueDate = time.Now().Add(14 * 24 * time.Hour).Format("2006-01-02") + "T00:00:00.0000000+01:00"
+			} else if !strings.Contains(dueDate, "T") {
+				dueDate = dueDate + "T00:00:00.0000000+01:00"
+			}
+			if qty == 0 {
+				qty = 1
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			out, err := c.CreateInvoice(cmd.Context(), onlyoffice.CreateInvoiceParams{
+				Number:              number,
+				IssueDate:           issueDate,
+				DueDate:             dueDate,
+				ContactID:           contactID,
+				ConsigneeID:         consigneeID,
+				EntityID:            opportunityID,
+				EntityType:          0, // Opportunity
+				Language:            language,
+				Currency:            currency,
+				ExchangeRate:        1,
+				PurchaseOrderNumber: po,
+				Terms:               terms,
+				Description:         description,
+				Lines: []onlyoffice.InvoiceLine{{
+					InvoiceItemID: itemID,
+					Description:   lineDesc,
+					Quantity:      qty,
+					Price:         price,
+					SortOrder:     0,
+				}},
+			})
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+	cmd.Flags().StringVar(&number, "number", "", "invoice number (e.g. INV-2026-01)")
+	cmd.Flags().Int64Var(&contactID, "contact", 0, "bill-to company contact id (canonical company, not a PDF-only clone)")
+	cmd.Flags().Int64Var(&consigneeID, "consignee", 0, "optional Empfänger person/contact id")
+	cmd.Flags().Int64Var(&opportunityID, "opportunity", 0, "link to CRM opportunity/deal id (set at create)")
+	cmd.Flags().Int64Var(&itemID, "item", 0, "catalog invoice item id")
+	cmd.Flags().Float64Var(&price, "price", 0, "line price")
+	cmd.Flags().Float64Var(&qty, "qty", 1, "line quantity")
+	cmd.Flags().StringVar(&lineDesc, "line-description", "", "line description override")
+	cmd.Flags().StringVar(&issueDate, "issue-date", "", "YYYY-MM-DD (default today)")
+	cmd.Flags().StringVar(&dueDate, "due-date", "", "YYYY-MM-DD (default +14d)")
+	cmd.Flags().StringVar(&language, "language", "de-DE", "invoice language")
+	cmd.Flags().StringVar(&currency, "currency", "EUR", "currency abbreviation")
+	cmd.Flags().StringVar(&terms, "terms", "", "payment terms / footer")
+	cmd.Flags().StringVar(&description, "description", "", "invoice description")
+	cmd.Flags().StringVar(&po, "po", "", "purchase order number")
+	return cmd
+}
+
+func invoiceUpdateCmd() *cobra.Command {
+	var opportunityID int64
+	var description, po, terms string
+	cmd := &cobra.Command{
+		Use:   "update INVOICE_ID",
+		Short: "Update invoice (notes, PO, terms; opportunity link unreliable)",
+		Long: `Update Draft invoice fields.
+
+--opportunity often returns HTTP 400 on existing invoices. Prefer
+oo invoices create … --opportunity, or delete+recreate. See docs/crm-associations.md.
+`,
+		Args: cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if opportunityID == 0 && !cmd.Flags().Changed("description") && !cmd.Flags().Changed("po") && !cmd.Flags().Changed("terms") {
+				return fmt.Errorf("set --opportunity and/or --description and/or --po and/or --terms")
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			out, err := c.UpdateInvoice(cmd.Context(), args[0], onlyoffice.UpdateInvoiceParams{
+				EntityID:         opportunityID,
+				EntityType:       0,
+				Description:      description,
+				DescriptionSet:   cmd.Flags().Changed("description"),
+				PurchaseOrder:    po,
+				PurchaseOrderSet: cmd.Flags().Changed("po"),
+				Terms:            terms,
+				TermsSet:         cmd.Flags().Changed("terms"),
+			})
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+	cmd.Flags().Int64Var(&opportunityID, "opportunity", 0, "CRM opportunity/deal id (may 400 — prefer create)")
+	cmd.Flags().StringVar(&description, "description", "", "invoice notes (Notizen); use \\n for line breaks")
+	cmd.Flags().StringVar(&po, "po", "", "purchase order number")
+	cmd.Flags().StringVar(&terms, "terms", "", "payment terms / footer (Bedingungen)")
+	return cmd
+}
+
+func invoicePDFCmd() *cobra.Command {
+	var force bool
+	cmd := &cobra.Command{
+		Use:   "pdf INVOICE_ID",
+		Short: "Return / regenerate invoice PDF file metadata",
+		Long: `GET /api/2.0/crm/invoice/{id}/pdf.
+
+With --force, touches the Draft to clear cached fileID first (layout changes).
+`,
+		Args: cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			var out map[string]any
+			if force {
+				out, err = c.ForceRegenerateInvoicePDF(cmd.Context(), args[0])
+			} else {
+				out, err = c.InvoicePDFFile(cmd.Context(), args[0])
+			}
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+	cmd.Flags().BoolVar(&force, "force", false, "clear PDF cache then regenerate")
+	return cmd
+}
+
+func invoicePDFCleanupCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "pdf-cleanup INVOICE_ID",
+		Short: "Delete older P-*.pdf copies on company/deal; keep invoice.fileID",
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			deleted, err := c.PurgeStaleInvoicePDFs(cmd.Context(), args[0])
+			if err != nil {
+				return err
+			}
+			if outputFormat == "json" {
+				printObject(map[string]any{"deleted": deleted})
+				return nil
+			}
+			fmt.Printf("deleted %d stale PDF file(s): %v\n", len(deleted), deleted)
+			return nil
+		},
+	}
+}
+
+func invoiceStatusCmd() *cobra.Command {
+	var status string
+	cmd := &cobra.Command{
+		Use:   "status INVOICE_ID [INVOICE_ID...]",
+		Short: "Set invoice status (draft|billed|rejected|paid)",
+		Long: `PUT /api/2.0/crm/invoice/status/{id}.
+
+Billed invoices are not content-editable. Billed→Draft often does not work —
+recreate as Draft instead (docs/crm-associations.md).
+`,
+		Args: cobra.MinimumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			statusID, err := parseInvoiceStatus(status)
+			if err != nil {
+				return err
+			}
+			ids := make([]int64, 0, len(args))
+			for _, a := range args {
+				var id int64
+				if _, err := fmt.Sscan(a, &id); err != nil || id <= 0 {
+					return fmt.Errorf("invalid invoice id %q", a)
+				}
+				ids = append(ids, id)
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			out, err := c.SetInvoiceStatus(cmd.Context(), statusID, ids...)
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+	cmd.Flags().StringVar(&status, "status", "draft", "draft|billed|rejected|paid or numeric id")
+	return cmd
+}
+
+func parseInvoiceStatus(s string) (int, error) {
+	s = strings.TrimSpace(strings.ToLower(s))
+	switch s {
+	case "", "draft", "1":
+		return onlyoffice.InvoiceStatusDraft, nil
+	case "billed", "2":
+		return onlyoffice.InvoiceStatusBilled, nil
+	case "rejected", "3":
+		return onlyoffice.InvoiceStatusRejected, nil
+	case "paid", "4":
+		return onlyoffice.InvoiceStatusPaid, nil
+	default:
+		var n int
+		if _, err := fmt.Sscan(s, &n); err != nil || n <= 0 {
+			return 0, fmt.Errorf("unknown status %q (draft|billed|rejected|paid)", s)
+		}
+		return n, nil
+	}
+}
+
+func invoiceDeleteCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "delete INVOICE_ID [INVOICE_ID...]",
+		Short: "Delete one or more invoices",
+		Args:  cobra.MinimumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			for _, id := range args {
+				if _, err := c.DeleteInvoice(cmd.Context(), id); err != nil {
+					return err
+				}
+				fmt.Println("deleted", id)
+			}
+			return nil
+		},
+	}
+}
+
+func invoiceItemsCmd() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:     "items",
+		Aliases: []string{"item"},
+		Short:   "Invoice catalog items",
+	}
+	cmd.AddCommand(invoiceItemsListCmd())
+	cmd.AddCommand(invoiceItemsCreateCmd())
+	cmd.AddCommand(invoiceItemsDeleteCmd())
+	return cmd
+}
+
+func invoiceItemsListCmd() *cobra.Command {
+	var count, offset int
+	cmd := &cobra.Command{
+		Use:   "list",
+		Short: "List catalog invoice items",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			list, total, err := c.ListInvoiceItems(cmd.Context(), count, offset)
+			if err != nil {
+				return err
+			}
+			if outputFormat == "table" {
+				fmt.Printf("total: %d (shown: %d)\n", total, len(list))
+				for _, row := range list {
+					if cur, ok := row["currency"].(map[string]any); ok {
+						row["currency"] = cur["abbreviation"]
+					}
+				}
+			}
+			printTable([]string{"id", "title", "price", "currency", "description"}, list)
+			return nil
+		},
+	}
+	cmd.Flags().IntVar(&count, "count", 50, "")
+	cmd.Flags().IntVar(&offset, "offset", 0, "")
+	return cmd
+}
+
+func invoiceItemsCreateCmd() *cobra.Command {
+	var title, description, currency string
+	var price float64
+	cmd := &cobra.Command{
+		Use:   "create",
+		Short: "Create a catalog invoice item",
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if title == "" {
+				return fmt.Errorf("--title is required")
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			out, err := c.CreateInvoiceItem(cmd.Context(), title, description, price, currency)
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+	cmd.Flags().StringVar(&title, "title", "", "item title")
+	cmd.Flags().StringVar(&description, "description", "", "item description")
+	cmd.Flags().Float64Var(&price, "price", 0, "unit price")
+	cmd.Flags().StringVar(&currency, "currency", "EUR", "currency")
+	return cmd
+}
+
+func invoiceItemsDeleteCmd() *cobra.Command {
+	return &cobra.Command{
+		Use:   "delete ITEM_ID [ITEM_ID...]",
+		Short: "Delete catalog invoice items",
+		Args:  cobra.MinimumNArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			for _, id := range args {
+				if _, err := c.DeleteInvoiceItem(cmd.Context(), id); err != nil {
+					return err
+				}
+				fmt.Println("deleted", id)
+			}
+			return nil
+		},
+	}
+}
diff --git a/cmd/oo/mails.go b/cmd/oo/mails.go
index 4e7f254..d6dc4fe 100644
--- a/cmd/oo/mails.go
+++ b/cmd/oo/mails.go
@@ -1,6 +1,7 @@
 package main
 
 import (
+	"fmt"
 	"strconv"
 	"strings"
 
@@ -11,7 +12,7 @@ import (
 var mailsCmd = &cobra.Command{
 	Use:     "mails",
 	Aliases: []string{"mail"},
-	Short:   "OnlyOffice Workspace mail — list, read, delete",
+	Short:   "OnlyOffice Workspace mail — list, read, draft, delete",
 }
 
 func init() {
@@ -20,6 +21,9 @@ func init() {
 	mailsCmd.AddCommand(mailsFoldersCmd())
 	mailsCmd.AddCommand(mailsListCmd())
 	mailsCmd.AddCommand(mailsGetCmd())
+	mailsCmd.AddCommand(mailsDraftCmd())
+	mailsCmd.AddCommand(mailsAttachCmd())
+	mailsCmd.AddCommand(mailsDraftInvoiceCmd())
 	mailsCmd.AddCommand(mailsDeleteCmd())
 }
 
@@ -126,6 +130,179 @@ func mailsGetCmd() *cobra.Command {
 	}
 }
 
+func mailsDraftCmd() *cobra.Command {
+	var from, to, cc, bcc, subject, body, html string
+	var id int64
+	cmd := &cobra.Command{
+		Use:   "draft",
+		Short: "Create or update a mail draft (OnlyOffice Mail)",
+		Long: `Save a draft in /addons/mail (PUT /api/2.0/mail/drafts/save).
+
+  oo mails draft --to a@b.com --subject "…" --body "<p>…</p>"
+  oo mails draft --id 123 --to a@b.com --subject "…" --html "<p>…</p>"
+`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if to == "" {
+				return fmt.Errorf("--to is required")
+			}
+			htmlBody := body
+			if html != "" {
+				htmlBody = html
+			}
+			if htmlBody == "" {
+				return fmt.Errorf("--body or --html is required")
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			out, err := c.SaveMailDraft(cmd.Context(), onlyoffice.SaveMailDraftParams{
+				ID:      id,
+				From:    from,
+				To:      to,
+				Cc:      cc,
+				Bcc:     bcc,
+				Subject: subject,
+				Body:    onlyoffice.PlainTextToMailHTML(htmlBody),
+			})
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+	cmd.Flags().Int64Var(&id, "id", 0, "existing draft id (0 = create)")
+	cmd.Flags().StringVar(&from, "from", "", "from address (default: first enabled mailbox)")
+	cmd.Flags().StringVar(&to, "to", "", "recipient (required)")
+	cmd.Flags().StringVar(&cc, "cc", "", "cc")
+	cmd.Flags().StringVar(&bcc, "bcc", "", "bcc")
+	cmd.Flags().StringVar(&subject, "subject", "", "subject")
+	cmd.Flags().StringVar(&body, "body", "", "plain text or HTML body")
+	cmd.Flags().StringVar(&html, "html", "", "HTML body (alias of --body when set)")
+	return cmd
+}
+
+func mailsAttachCmd() *cobra.Command {
+	var fileID int64
+	cmd := &cobra.Command{
+		Use:   "attach MESSAGE_ID",
+		Short: "Attach an OnlyOffice Files document to a draft/message",
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if fileID <= 0 {
+				return fmt.Errorf("--file-id is required")
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			out, err := c.AttachMailDocument(cmd.Context(), args[0], fileID)
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+	cmd.Flags().Int64Var(&fileID, "file-id", 0, "OnlyOffice file id (e.g. invoice PDF)")
+	return cmd
+}
+
+func mailsDraftInvoiceCmd() *cobra.Command {
+	var to, from, subject, body string
+	var invoiceID int
+	cmd := &cobra.Command{
+		Use:   "draft-invoice",
+		Short: "Create a mail draft with regenerated invoice PDF attached",
+		Long: `Regenerate the CRM invoice PDF, save an OnlyOffice Mail draft, and attach the PDF.
+
+Does not send. Open /addons/mail/#drafts to review.
+
+  oo mails draft-invoice --invoice 16 --to info@example.com
+`,
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if invoiceID <= 0 || to == "" {
+				return fmt.Errorf("--invoice and --to are required")
+			}
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			pdf, err := c.ForceRegenerateInvoicePDF(cmd.Context(), strconv.Itoa(invoiceID))
+			if err != nil {
+				return err
+			}
+			fileID := onlyoffice.Int64FromMap(pdf, "id")
+			if fileID <= 0 {
+				return fmt.Errorf("invoice PDF has no file id: %+v", pdf)
+			}
+			inv, err := c.GetInvoice(cmd.Context(), strconv.Itoa(invoiceID))
+			if err != nil {
+				return err
+			}
+			number := strings.TrimSpace(fmt.Sprint(inv["number"]))
+			cost := formatInvoiceCostEUR(inv["cost"])
+			if subject == "" {
+				subject = fmt.Sprintf("Rechnung %s — Service package (%s EUR)", number, cost)
+			}
+			if body == "" {
+				body = fmt.Sprintf(`Guten Tag,
+
+anbei übersende ich die Rechnung %s über %s EUR
+für das Service package (OS image inkl. Backup, Monitoring, RustDesk,
+gemeinsamem Datenträger und Netzwerkzugriff).
+
+Zahlungsziel: 14 Tage netto.
+
+Steuerhinweis: siehe Notizen auf der Rechnung
+(Reverse Charge / § 13b UStG).
+
+Mit freundlichen Grüßen
+
+Example Author`, number, cost)
+			}
+			draft, err := c.SaveMailDraft(cmd.Context(), onlyoffice.SaveMailDraftParams{
+				From:    from,
+				To:      to,
+				Subject: subject,
+				Body:    onlyoffice.MailHTMLWithBlankParagraphs(onlyoffice.PlainTextToMailHTML(body)),
+			})
+			if err != nil {
+				return err
+			}
+			mid := fmt.Sprint(draft["id"])
+			att, err := c.AttachMailDocument(cmd.Context(), mid, fileID)
+			if err != nil {
+				return fmt.Errorf("draft %s created but attach failed: %w", mid, err)
+			}
+			if outputFormat == "json" {
+				printObject(map[string]any{"draft": draft, "attachment": att, "pdfFileId": fileID})
+				return nil
+			}
+			fmt.Printf("draft %s  to=%s  subject=%q  pdfFileId=%d\n", mid, to, subject, fileID)
+			fmt.Printf("open: https://office.example.com/addons/mail/#drafts\n")
+			return nil
+		},
+	}
+	cmd.Flags().IntVar(&invoiceID, "invoice", 0, "CRM invoice id")
+	cmd.Flags().StringVar(&to, "to", "", "recipient")
+	cmd.Flags().StringVar(&from, "from", "", "from address (default mailbox)")
+	cmd.Flags().StringVar(&subject, "subject", "", "override subject")
+	cmd.Flags().StringVar(&body, "body", "", "override plain-text body")
+	return cmd
+}
+
+func formatInvoiceCostEUR(v any) string {
+	s := strings.TrimSpace(fmt.Sprint(v))
+	s = strings.TrimSuffix(s, ".00")
+	s = strings.TrimSuffix(s, ".0")
+	if s == "" || s == "<nil>" {
+		return "?"
+	}
+	return s
+}
+
 func mailsDeleteCmd() *cobra.Command {
 	return &cobra.Command{
 		Use:     "delete ID [ID...]",
diff --git a/cmd/oo/main.go b/cmd/oo/main.go
index 4300879..155cebe 100644
--- a/cmd/oo/main.go
+++ b/cmd/oo/main.go
@@ -6,17 +6,20 @@
 //	oo projects      list | get | milestones | create | update | delete | files (list|upload|download|rename|delete)
 //	oo tasks         list | get | create | update | delete | subtask add | files (list|upload|detach)
 //	oo users         list | self            (alias: oo whoami)
-//	oo contacts      list | get | delete | info-add | dedupe-info
+//	oo contacts      list | get | delete | info-add | merge | dedupe-info
 //	oo persons       list | create | delete | dedupe
 //	oo companies     list | create | delete | dedupe | dedupe-persons
 //	oo opportunities list | get | create | delete | stages | member-add | dedupe | dedupe-members | fix-titles
 //	oo cases         list | create | delete | member-add
 //	oo crm-tasks     list | create | delete | categories
 //	oo crm           cleanup
-//	oo mails         accounts | folders | list | get | delete
+//	oo mails         accounts | folders | list | get | draft | attach | draft-invoice | delete
+//	oo invoices      list | get | create | update | pdf | pdf-cleanup | status | delete | items …
 //	oo applications  sync
 //	oo catalog       scan-contacts | scan-projects | scan-thunderbird | merge | match | apply
 //
+// CRM association rules: docs/crm-associations.md
+//
 // Every list supports `--output/-o json|table` (table is the default).
 //
 // Build & install:
diff --git a/cmd/oo/opportunities.go b/cmd/oo/opportunities.go
index b20e8e5..b65632f 100644
--- a/cmd/oo/opportunities.go
+++ b/cmd/oo/opportunities.go
@@ -19,6 +19,7 @@ func init() {
 	opportunitiesCmd.AddCommand(oppListCmd())
 	opportunitiesCmd.AddCommand(oppGetCmd())
 	opportunitiesCmd.AddCommand(oppCreateCmd())
+	opportunitiesCmd.AddCommand(oppUpdateCmd())
 	opportunitiesCmd.AddCommand(oppDeleteCmd())
 	opportunitiesCmd.AddCommand(oppStagesCmd())
 	opportunitiesCmd.AddCommand(oppMemberAddCmd())
@@ -118,6 +119,40 @@ func oppCreateCmd() *cobra.Command {
 	return cmd
 }
 
+func oppUpdateCmd() *cobra.Command {
+	var title, desc string
+	var stage int64
+	var bid float64
+	cmd := &cobra.Command{
+		Use:   "update OPPORTUNITY_ID",
+		Short: "Update opportunity title, description, stage, or bid",
+		Args:  cobra.ExactArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			c, err := newOO(cmd)
+			if err != nil {
+				return err
+			}
+			out, err := c.UpdateOpportunity(cmd.Context(), args[0], onlyoffice.UpdateOpportunityParams{
+				Title:       title,
+				Description: desc,
+				StageID:     stage,
+				BidValue:    bid,
+				BidValueSet: cmd.Flags().Changed("bid"),
+			})
+			if err != nil {
+				return err
+			}
+			printObject(out)
+			return nil
+		},
+	}
+	cmd.Flags().StringVar(&title, "title", "", "new title")
+	cmd.Flags().StringVar(&desc, "description", "", "new description")
+	cmd.Flags().Int64Var(&stage, "stage", 0, "pipeline stage id")
+	cmd.Flags().Float64Var(&bid, "bid", 0, "bid value")
+	return cmd
+}
+
 func oppDeleteCmd() *cobra.Command {
 	return &cobra.Command{
 		Use:     "delete OPPORTUNITY_ID [OPPORTUNITY_ID...]",
diff --git a/docs/crm-associations.md b/docs/crm-associations.md
new file mode 100644
index 0000000..2405561
--- /dev/null
+++ b/docs/crm-associations.md
@@ -0,0 +1,121 @@
+# CRM associations (company ↔ person ↔ deal ↔ project ↔ invoice ↔ mail)
+
+Operational rules learned from real billing work (AcmeClient INV-2026-01, AcmeProject INV-2026-01).
+Tooling: `oo` CLI + this library. Business SSOT remains OnlyOffice (`ASR-0002`).
+
+## Canonical graph
+
+One **legal company** owns the relationship. Do not invent a second “bill-to”
+company just for PDF layout.
+
+```text
+Company (#CONTACT_ID)
+├── Person (GF / buyer contact)     oo persons create --company-id
+├── Opportunity / Deal              oo opportunities … ; member-add company + person
+├── Project (hub)                   oo projects … ; contacts add company + person
+│     └── Epic + subtasks
+└── Invoice (Draft → …)             oo invoices create --contact COMPANY --opportunity DEAL
+      └── PDF file                  oo invoices pdf ID
+            └── Mail draft          oo mails draft-invoice --invoice ID --to …
+```
+
+| Layer | CLI | Must link |
+|-------|-----|-----------|
+| Company | `oo companies create` | website, email, phone, **one** Billing address |
+| Person | `oo persons create --company-id` | job title; never encode employer in `lastName` |
+| Deal | `oo opportunities create` + `member-add` | company **and** person as members |
+| Project | `oo projects create` + `contacts add` | same company + person |
+| Invoice | `oo invoices create --contact COMPANY --opportunity DEAL` | `entityId` at **create** |
+| Mail | `oo mails draft-invoice` | attach current PDF; **do not send** until confirmed |
+
+UI checks (same company card):
+
+- `#contacts` → person  
+- `#deals` → opportunity  
+- `#projects` → hub project  
+- `#invoices` on the **deal** → invoice (needs `entity`)  
+- `#files` → preferably **one** current `P-….pdf`
+
+## Hard rules
+
+1. **One company per legal entity.** Duplicate “bill-to” contacts empty Deals /
+   Projects / Contacts tabs and break merge. Prefer
+   `oo contacts merge FROM INTO` (keeps `INTO`) or `oo companies dedupe`.
+2. **Link invoice → deal at create.**  
+   `POST /crm/invoice` with `entityId` + `entityType: 0` (Opportunity).  
+   `oo invoices update … --opportunity` often returns **400**
+   (“Value does not fall within the expected range”). If the link is missing,
+   delete the Draft and recreate with `--opportunity`.
+3. **Bill To = company id**, not a throwaway contact. Person stays under the
+   company (`companyId`). Optional `consigneeId` for Empfänger when the portal
+   template prints it.
+4. **Stay Draft until mail is ready.** Billed (`status id=2`) is **not editable**
+   via content PUT. Going Billed → Draft via `…/crm/invoice/status/1` usually
+   **does not work** — delete + recreate Draft instead.
+5. **Do not regenerate PDF in a loop** without cleanup. Each
+   `GET …/crm/invoice/{id}/pdf` attaches a new file to the company (and often
+   the deal). Keep `invoice.fileID`; delete older `P-*.pdf` with
+   `oo invoices pdf-cleanup ID` / Documents `fileops/delete`.
+
+## Invoice PDF quirks
+
+| Symptom | Workaround |
+|---------|------------|
+| Cached / stale PDF | Touch invoice (Draft PUT that clears `fileID`), then `GET …/pdf` — `oo invoices pdf ID --force` |
+| Billing address missing on **new** PDFs | Temporary multiline `companyName` (`Line1\nLine2\n…`) on the **canonical** company → force PDF → restore clean name. Cached `fileID` keeps the multiline Bill To. |
+| Separate bill-to company for newlines | **Forbidden** — merge back to the real company |
+| Invoice **number** won’t change on PUT | Delete Draft and recreate with the desired number |
+| Notizen / Bedingungen spacing | Leading `\n` and blank lines only — no HTML (tags print literally) |
+| Issuer NIE / street lines | Organisation profile address (`street` with `\n`), not only terms |
+
+Status ids commonly used on this portal: `1` Draft, `2` Billed, `3` Rejected, `4` Paid.
+
+## Mail quirks
+
+| Symptom | Workaround |
+|---------|------------|
+| Signature / body doubles Matrix URL | Put chat in **one** place only. UI drafts: signature. API send: body (API **does not** append signature). |
+| Signature / body cuts Matrix URL at `#` | Plain text `chat: https://matrix.to/#/@user:server` — avoid `<a href="…#…">` (or encode `#` as `%23` in href) |
+| German letter spacing | Blank `<p>&nbsp;</p>` between blocks (`MailHTMLWithBlankParagraphs`) |
+| Send | `PUT /api/2.0/mail/messages/send.json` with `id/from/to/subject/body`; omit empty `cc`/`bcc`. Never auto-send; draft only until the human confirms |
+
+Prefer OnlyOffice Mail (`/addons/mail/#drafts`) over Gmail MCP for invoice delivery.
+
+## Project / task quirks
+
+- Hub title: `CC | Company` (e.g. `DE | AcmeClient Ambulanter Pflegedienst GmbH`).
+- Streams = epics/tasks under the hub, not a third title segment (unless the
+  project itself is a named delivery stream).
+- Closing a **subtask**:  
+  `PUT /api/2.0/project/task/{epicId}/{subtaskId}/status` with `status=2`.  
+  `oo tasks update SUBTASK -s closed` returns **404** for subtasks.
+- After deleting a CRM contact, `GET /project/contact/{deletedId}` may still
+  return projects (ghost). Official project contact list should only show live
+  ids; unlink may 400 if the contact is gone.
+
+## Merge / cleanup cheat sheet
+
+```bash
+# Keep the human-created company (INTO), drop the duplicate (FROM)
+oo contacts merge 1334 1328
+
+# Or by normalized name (careful — whole CRM)
+oo companies dedupe
+
+# Invoice ↔ deal must exist at create
+oo invoices create --number P-YYYY-NN --contact CONTACT_ID --item N \
+  --price 300 --opportunity OPPORTUNITY_ID --language de-DE …
+
+# Fresh PDF + prune older P-*.pdf on company/deal
+oo invoices pdf 41 --force
+oo invoices pdf-cleanup INVOICE_ID
+
+# Mail draft (no send)
+oo mails draft-invoice --invoice INVOICE_ID --to info@client.de
+```
+
+## Related
+
+- README § invoices / mail / CRM cleanup  
+- Skill `oo-clients-projects` (Cursor)  
+- inventar ASR-0014 / `ops/oo-clients-contacts-sync.md`
diff --git a/http.go b/http.go
index 3d94999..570286f 100644
--- a/http.go
+++ b/http.go
@@ -244,6 +244,58 @@ func (c *Client) putJSONObject(ctx context.Context, path string, body any) (map[
 	return unmarshalResponseObject(raw)
 }
 
+// postJSONObject issues an authenticated POST with JSON body and decodes response.
+func (c *Client) postJSONObject(ctx context.Context, path string, body any) (map[string]any, error) {
+	raw, err := c.postJSON(ctx, path, body)
+	if err != nil {
+		return nil, err
+	}
+	return unmarshalResponseObject(raw)
+}
+
+// postJSON issues an authenticated POST with application/json body.
+func (c *Client) postJSON(ctx context.Context, path string, body any) (json.RawMessage, error) {
+	auth, err := c.authHeader()
+	if err != nil {
+		return nil, err
+	}
+	var rdr io.Reader
+	switch b := body.(type) {
+	case nil:
+		rdr = strings.NewReader("{}")
+	case []byte:
+		rdr = bytes.NewReader(b)
+	case string:
+		rdr = strings.NewReader(b)
+	default:
+		buf, err := json.Marshal(b)
+		if err != nil {
+			return nil, err
+		}
+		rdr = bytes.NewReader(buf)
+	}
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, c.baseURL()+path, rdr)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Authorization", auth)
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Accept", "application/json")
+	resp, err := c.client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	raw, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, err
+	}
+	if resp.StatusCode >= 400 {
+		return nil, fmt.Errorf("POST JSON %s: %d %s", path, resp.StatusCode, truncate(string(raw), 400))
+	}
+	return raw, nil
+}
+
 // putJSON issues an authenticated PUT with application/json body.
 func (c *Client) putJSON(ctx context.Context, path string, body any) (json.RawMessage, error) {
 	auth, err := c.authHeader()
diff --git a/invoices.go b/invoices.go
new file mode 100644
index 0000000..6c9cb1d
--- /dev/null
+++ b/invoices.go
@@ -0,0 +1,545 @@
+package onlyoffice
+
+// CRM Invoices and invoice catalog items (billing).
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/url"
+	"strconv"
+	"strings"
+)
+
+// InvoiceLine is one line on a create-invoice request.
+type InvoiceLine struct {
+	InvoiceItemID int64   `json:"invoiceItemID"`
+	InvoiceTax1ID int64   `json:"invoiceTax1ID"`
+	InvoiceTax2ID int64   `json:"invoiceTax2ID"`
+	Description   string  `json:"description"`
+	Quantity      float64 `json:"quantity"`
+	Price         float64 `json:"price"`
+	Discount      float64 `json:"discount"`
+	SortOrder     int     `json:"sortOrder"`
+}
+
+// CreateInvoiceParams holds fields for POST /api/2.0/crm/invoice.
+type CreateInvoiceParams struct {
+	Number              string
+	IssueDate           string // ISO-8601
+	DueDate             string // ISO-8601
+	ContactID           int64
+	ConsigneeID         int64 // 0 = omit
+	EntityID            int64 // 0 = omit; link to opportunity/case
+	EntityType          int   // OnlyOffice EntityType; 0 = Opportunity
+	Language            string
+	Currency            string
+	ExchangeRate        float64
+	PurchaseOrderNumber string
+	Terms               string
+	Description         string
+	TemplateType        int
+	Lines               []InvoiceLine
+}
+
+// ListInvoices returns a page of CRM invoices and the total count.
+func (c *Client) ListInvoices(ctx context.Context, count, startIndex int) ([]map[string]any, int, error) {
+	q := url.Values{}
+	q.Set("count", strconv.Itoa(count))
+	q.Set("startIndex", strconv.Itoa(startIndex))
+	raw, err := c.getJSON(ctx, "/api/2.0/crm/invoice/filter.json?"+q.Encode())
+	if err != nil {
+		return nil, 0, err
+	}
+	var env struct {
+		Response []map[string]any `json:"response"`
+		Total    int              `json:"total"`
+	}
+	if err := json.Unmarshal(raw, &env); err != nil {
+		return nil, 0, err
+	}
+	total := env.Total
+	if total == 0 && len(env.Response) > 0 {
+		total = len(env.Response)
+	}
+	return env.Response, total, nil
+}
+
+// GetInvoice returns a single invoice by id (includes invoiceLines).
+func (c *Client) GetInvoice(ctx context.Context, id string) (map[string]any, error) {
+	return c.ResponseObject(ctx, fmt.Sprintf("/api/2.0/crm/invoice/%s.json", url.PathEscape(id)))
+}
+
+// CreateInvoice creates a draft invoice with the given lines.
+func (c *Client) CreateInvoice(ctx context.Context, p CreateInvoiceParams) (map[string]any, error) {
+	if p.ContactID == 0 {
+		return nil, fmt.Errorf("contactId is required")
+	}
+	if p.Number == "" {
+		return nil, fmt.Errorf("number is required")
+	}
+	if len(p.Lines) == 0 {
+		return nil, fmt.Errorf("at least one invoice line is required")
+	}
+	if p.Language == "" {
+		p.Language = "de-DE"
+	}
+	if p.Currency == "" {
+		p.Currency = "EUR"
+	}
+	if p.ExchangeRate == 0 {
+		p.ExchangeRate = 1
+	}
+	body := map[string]any{
+		"number":              p.Number,
+		"issueDate":           p.IssueDate,
+		"dueDate":             p.DueDate,
+		"contactId":           p.ContactID,
+		"language":            p.Language,
+		"currency":            p.Currency,
+		"exchangeRate":        p.ExchangeRate,
+		"purchaseOrderNumber": p.PurchaseOrderNumber,
+		"terms":               p.Terms,
+		"description":         p.Description,
+		"templateType":        p.TemplateType,
+		"invoiceLines":        p.Lines,
+	}
+	if p.ConsigneeID != 0 {
+		body["consigneeId"] = p.ConsigneeID
+	}
+	if p.EntityID != 0 {
+		body["entityId"] = p.EntityID
+		body["entityType"] = p.EntityType // 0 = Opportunity on this portal
+	}
+	return c.postJSONObject(ctx, "/api/2.0/crm/invoice", body)
+}
+
+// UpdateInvoiceParams are fields for PUT /api/2.0/crm/invoice/{id}.
+// Loads the current invoice and merges non-zero / set fields.
+type UpdateInvoiceParams struct {
+	EntityID         int64 // link to opportunity; 0 = leave unchanged
+	EntityType       int
+	ContactID        int64  // 0 = leave unchanged
+	Description      string // invoice notes; empty + DescriptionSet=false keeps existing
+	DescriptionSet   bool
+	PurchaseOrder    string
+	PurchaseOrderSet bool
+	Terms            string
+	TermsSet         bool
+}
+
+// UpdateInvoice PUTs a full invoice body (OnlyOffice requires complete payload).
+//
+// Linking an opportunity via EntityID on an existing invoice often returns HTTP 400
+// on this portal — prefer CreateInvoice with EntityID set. See docs/crm-associations.md.
+func (c *Client) UpdateInvoice(ctx context.Context, id string, p UpdateInvoiceParams) (map[string]any, error) {
+	inv, err := c.GetInvoice(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	contactID := int64(0)
+	if m, ok := inv["contact"].(map[string]any); ok {
+		contactID = flexInt(m["id"])
+	}
+	if p.ContactID != 0 {
+		contactID = p.ContactID
+	}
+	cur := "EUR"
+	if m, ok := inv["currency"].(map[string]any); ok {
+		if a := stringField(m, "abbreviation"); a != "" {
+			cur = a
+		}
+	}
+	statusID := 1
+	if m, ok := inv["status"].(map[string]any); ok {
+		statusID = int(flexInt(m["id"]))
+	}
+	var lines []map[string]any
+	switch raw := inv["invoiceLines"].(type) {
+	case []any:
+		for _, row := range raw {
+			m, ok := row.(map[string]any)
+			if !ok {
+				continue
+			}
+			lines = append(lines, map[string]any{
+				"id":            flexInt(m["id"]),
+				"invoiceItemID": flexInt(m["invoiceItemID"]),
+				"invoiceTax1ID": flexInt(m["invoiceTax1ID"]),
+				"invoiceTax2ID": flexInt(m["invoiceTax2ID"]),
+				"description":   stringField(m, "description"),
+				"quantity":      floatField(m, "quantity"),
+				"price":         floatField(m, "price"),
+				"discount":      floatField(m, "discount"),
+				"sortOrder":     int(flexInt(m["sortOrder"])),
+			})
+		}
+	case []map[string]any:
+		for _, m := range raw {
+			lines = append(lines, map[string]any{
+				"id":            flexInt(m["id"]),
+				"invoiceItemID": flexInt(m["invoiceItemID"]),
+				"invoiceTax1ID": flexInt(m["invoiceTax1ID"]),
+				"invoiceTax2ID": flexInt(m["invoiceTax2ID"]),
+				"description":   stringField(m, "description"),
+				"quantity":      floatField(m, "quantity"),
+				"price":         floatField(m, "price"),
+				"discount":      floatField(m, "discount"),
+				"sortOrder":     int(flexInt(m["sortOrder"])),
+			})
+		}
+	}
+	body := map[string]any{
+		"id":                  flexInt(inv["id"]),
+		"number":              stringField(inv, "number"),
+		"issueDate":           stringField(inv, "issueDate"),
+		"dueDate":             stringField(inv, "dueDate"),
+		"contactId":           contactID,
+		"language":            stringField(inv, "language"),
+		"currency":            cur,
+		"exchangeRate":        floatField(inv, "exchangeRate"),
+		"purchaseOrderNumber": stringField(inv, "purchaseOrderNumber"),
+		"terms":               stringField(inv, "terms"),
+		"description":         stringField(inv, "description"),
+		"templateType":        int(flexInt(inv["templateType"])),
+		"status":              statusID,
+		"invoiceLines":        lines,
+	}
+	if p.DescriptionSet {
+		body["description"] = p.Description
+	}
+	if p.PurchaseOrderSet {
+		body["purchaseOrderNumber"] = p.PurchaseOrder
+	}
+	if p.TermsSet {
+		body["terms"] = p.Terms
+	}
+	if p.EntityID != 0 {
+		body["entityId"] = p.EntityID
+		body["entityType"] = p.EntityType
+	} else if ent, ok := inv["entity"].(map[string]any); ok && ent != nil {
+		body["entityId"] = flexInt(ent["entityId"])
+		// API returns entityType as string ("opportunity"); create/update want int.
+		switch v := ent["entityType"].(type) {
+		case float64:
+			body["entityType"] = int(v)
+		case int:
+			body["entityType"] = v
+		case string:
+			if strings.EqualFold(v, "opportunity") {
+				body["entityType"] = 0
+			}
+		}
+	}
+	return c.putJSONObject(ctx, fmt.Sprintf("/api/2.0/crm/invoice/%s", url.PathEscape(id)), body)
+}
+
+// DeleteInvoice removes an invoice by id.
+func (c *Client) DeleteInvoice(ctx context.Context, id string) (map[string]any, error) {
+	return c.deleteObject(ctx, fmt.Sprintf("/api/2.0/crm/invoice/%s.json", url.PathEscape(id)))
+}
+
+// Invoice status ids used by OnlyOffice CRM on produktor.io.
+const (
+	InvoiceStatusDraft    = 1
+	InvoiceStatusBilled   = 2
+	InvoiceStatusRejected = 3
+	InvoiceStatusPaid     = 4
+)
+
+// SetInvoiceStatus sets CRM invoice status for one or more invoice ids
+// (PUT /api/2.0/crm/invoice/status/{statusId} with invoiceids).
+// Note: Billed→Draft often does not stick; recreate Draft instead (see docs/crm-associations.md).
+func (c *Client) SetInvoiceStatus(ctx context.Context, statusID int, invoiceIDs ...int64) (map[string]any, error) {
+	if statusID <= 0 {
+		return nil, fmt.Errorf("status id is required")
+	}
+	if len(invoiceIDs) == 0 {
+		return nil, fmt.Errorf("at least one invoice id is required")
+	}
+	ids := make([]string, 0, len(invoiceIDs))
+	for _, id := range invoiceIDs {
+		ids = append(ids, strconv.FormatInt(id, 10))
+	}
+	fields := url.Values{}
+	fields.Set("invoiceids", strings.Join(ids, ","))
+	return c.putFormObject(ctx, fmt.Sprintf("/api/2.0/crm/invoice/status/%d", statusID), fields)
+}
+
+// InvoicePDFFile returns invoice PDF file metadata (id, title, viewUrl).
+// Without force, OnlyOffice may return a cached fileID with stale layout.
+func (c *Client) InvoicePDFFile(ctx context.Context, invoiceID string) (map[string]any, error) {
+	id := strings.TrimSpace(invoiceID)
+	if id == "" {
+		return nil, fmt.Errorf("InvoicePDFFile: invoice id is required")
+	}
+	return c.ResponseObject(ctx, fmt.Sprintf("/api/2.0/crm/invoice/%s/pdf", url.PathEscape(id)))
+}
+
+// ForceRegenerateInvoicePDF clears the cached PDF (Draft touch) then requests a new file.
+// No-op touch when the invoice is not editable (e.g. Billed) — falls back to GET /pdf.
+func (c *Client) ForceRegenerateInvoicePDF(ctx context.Context, invoiceID string) (map[string]any, error) {
+	id := strings.TrimSpace(invoiceID)
+	if id == "" {
+		return nil, fmt.Errorf("ForceRegenerateInvoicePDF: invoice id is required")
+	}
+	inv, err := c.GetInvoice(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	canEdit := true
+	if v, ok := inv["canEdit"].(bool); ok {
+		canEdit = v
+	}
+	if canEdit {
+		desc := stringField(inv, "description")
+		// Append/remove a trailing space so PUT clears fileID without visible change.
+		touch := desc + " "
+		if strings.HasSuffix(desc, " ") {
+			touch = strings.TrimSuffix(desc, " ")
+		}
+		if _, err := c.UpdateInvoice(ctx, id, UpdateInvoiceParams{
+			Description:    touch,
+			DescriptionSet: true,
+		}); err != nil {
+			return nil, fmt.Errorf("ForceRegenerateInvoicePDF: clear cache: %w", err)
+		}
+	}
+	return c.InvoicePDFFile(ctx, id)
+}
+
+// ListCRMContactFiles lists Documents attached on a CRM contact card (#files).
+func (c *Client) ListCRMContactFiles(ctx context.Context, contactID string) ([]map[string]any, error) {
+	return c.ResponseArray(ctx, fmt.Sprintf("/api/2.0/crm/contact/%s/files.json", url.PathEscape(contactID)))
+}
+
+// ListOpportunityFiles lists Documents attached on a CRM opportunity.
+func (c *Client) ListOpportunityFiles(ctx context.Context, opportunityID string) ([]map[string]any, error) {
+	return c.ResponseArray(ctx, fmt.Sprintf("/api/2.0/crm/opportunity/%s/files.json", url.PathEscape(opportunityID)))
+}
+
+// PurgeStaleInvoicePDFs deletes older P-*.pdf copies on the invoice contact (and linked
+// opportunity) while keeping the invoice's current fileID. Returns deleted file ids.
+func (c *Client) PurgeStaleInvoicePDFs(ctx context.Context, invoiceID string) ([]int, error) {
+	inv, err := c.GetInvoice(ctx, invoiceID)
+	if err != nil {
+		return nil, err
+	}
+	keep := flexInt(inv["fileID"])
+	number := strings.TrimSpace(stringField(inv, "number"))
+	base := number
+	for strings.HasSuffix(base, "b") || strings.HasSuffix(base, "B") {
+		base = base[:len(base)-1]
+	}
+	if base == "" {
+		base = "P-"
+	}
+
+	seen := map[int]struct{}{}
+	var candidates []int
+	addFiles := func(files []map[string]any) {
+		for _, f := range files {
+			title := stringField(f, "title")
+			id := int(flexInt(f["id"]))
+			if id == 0 || id == int(keep) {
+				continue
+			}
+			if !strings.HasPrefix(title, "P-") {
+				continue
+			}
+			if !strings.HasPrefix(title, base) {
+				continue
+			}
+			if _, ok := seen[id]; ok {
+				continue
+			}
+			seen[id] = struct{}{}
+			candidates = append(candidates, id)
+		}
+	}
+
+	if m, ok := inv["contact"].(map[string]any); ok {
+		cid := strconv.FormatInt(flexInt(m["id"]), 10)
+		if cid != "0" {
+			files, err := c.ListCRMContactFiles(ctx, cid)
+			if err != nil {
+				return nil, err
+			}
+			addFiles(files)
+		}
+	}
+	if ent, ok := inv["entity"].(map[string]any); ok && ent != nil {
+		if strings.EqualFold(fmt.Sprint(ent["entityType"]), "opportunity") || flexInt(ent["entityType"]) == 0 {
+			oid := strconv.FormatInt(flexInt(ent["entityId"]), 10)
+			if oid != "0" {
+				files, err := c.ListOpportunityFiles(ctx, oid)
+				if err != nil {
+					return nil, err
+				}
+				addFiles(files)
+			}
+		}
+	}
+
+	if len(candidates) == 0 {
+		return nil, nil
+	}
+	if err := c.DeleteFiles(ctx, candidates); err != nil {
+		return nil, err
+	}
+	// CRM may still list deleted files briefly; also try CRM unlink.
+	for _, fid := range candidates {
+		_, _ = c.deleteObject(ctx, fmt.Sprintf("/api/2.0/crm/files/%d.json", fid))
+	}
+	return candidates, nil
+}
+
+// ListInvoiceItems returns catalog invoice items.
+func (c *Client) ListInvoiceItems(ctx context.Context, count, startIndex int) ([]map[string]any, int, error) {
+	q := url.Values{}
+	q.Set("count", strconv.Itoa(count))
+	q.Set("startIndex", strconv.Itoa(startIndex))
+	raw, err := c.getJSON(ctx, "/api/2.0/crm/invoiceitem/filter.json?"+q.Encode())
+	if err != nil {
+		return nil, 0, err
+	}
+	var env struct {
+		Response []map[string]any `json:"response"`
+		Total    int              `json:"total"`
+	}
+	if err := json.Unmarshal(raw, &env); err != nil {
+		return nil, 0, err
+	}
+	total := env.Total
+	if total == 0 && len(env.Response) > 0 {
+		total = len(env.Response)
+	}
+	return env.Response, total, nil
+}
+
+// CreateInvoiceItem creates a reusable catalog line item.
+func (c *Client) CreateInvoiceItem(ctx context.Context, title, description string, price float64, currency string) (map[string]any, error) {
+	if title == "" {
+		return nil, fmt.Errorf("title is required")
+	}
+	if currency == "" {
+		currency = "EUR"
+	}
+	fields := url.Values{}
+	fields.Set("title", title)
+	fields.Set("description", description)
+	fields.Set("price", strconv.FormatFloat(price, 'f', 2, 64))
+	fields.Set("currency", currency)
+	fields.Set("stockKeepingUnit", "")
+	fields.Set("trackInventory", "false")
+	fields.Set("quantity", "0")
+	fields.Set("invoiceTax1ID", "0")
+	fields.Set("invoiceTax2ID", "0")
+	return c.postFormObject(ctx, "/api/2.0/crm/invoiceitem", fields)
+}
+
+// DeleteInvoiceItem removes a catalog invoice item.
+func (c *Client) DeleteInvoiceItem(ctx context.Context, id string) (map[string]any, error) {
+	return c.deleteObject(ctx, fmt.Sprintf("/api/2.0/crm/invoiceitem/%s.json", url.PathEscape(id)))
+}
+
+// AddContactAddress attaches a postal address to a contact.
+// category: Home|Postal|Office|Billing|Other|Work (or numeric string).
+func (c *Client) AddContactAddress(ctx context.Context, contactID, street, city, state, zip, country, category string, isPrimary bool) (map[string]any, error) {
+	if category == "" {
+		category = "Billing"
+	}
+	fields := url.Values{}
+	fields.Set("street", street)
+	fields.Set("city", city)
+	fields.Set("state", state)
+	fields.Set("zip", zip)
+	fields.Set("country", country)
+	fields.Set("category", category)
+	fields.Set("isPrimary", strconv.FormatBool(isPrimary))
+	return c.postFormObject(ctx, fmt.Sprintf("/api/2.0/crm/contact/%s/address", url.PathEscape(contactID)), fields)
+}
+
+// UpdateCompany updates company name and optional about text.
+func (c *Client) UpdateCompany(ctx context.Context, companyID, name, about string) (map[string]any, error) {
+	fields := url.Values{}
+	fields.Set("companyName", name)
+	if about != "" {
+		fields.Set("about", about)
+	}
+	return c.putFormObject(ctx, fmt.Sprintf("/api/2.0/crm/contact/company/%s.json", url.PathEscape(companyID)), fields)
+}
+
+// UpdateOpportunityParams are optional fields for UpdateOpportunity.
+// Zero/empty values mean "keep existing" except BidValueSet.
+type UpdateOpportunityParams struct {
+	Title       string
+	Description string
+	StageID     int64
+	BidValue    float64
+	BidValueSet bool
+}
+
+// UpdateOpportunity loads a deal and PUTs updated fields.
+func (c *Client) UpdateOpportunity(ctx context.Context, id string, p UpdateOpportunityParams) (map[string]any, error) {
+	opp, err := c.GetOpportunity(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	title := p.Title
+	if title == "" {
+		title = stringField(opp, "title")
+	}
+	body := opportunityUpdateBody(opp, title)
+	if p.Description != "" {
+		body["description"] = p.Description
+	}
+	if p.StageID != 0 {
+		body["stageid"] = p.StageID
+	}
+	if p.BidValueSet {
+		body["bidValue"] = p.BidValue
+	}
+	return c.putJSONObject(ctx, fmt.Sprintf("/api/2.0/crm/opportunity/%s.json", url.PathEscape(id)), body)
+}
+
+// InvoiceContactName extracts displayName from nested contact on an invoice row.
+func InvoiceContactName(inv map[string]any) string {
+	if m, ok := inv["contact"].(map[string]any); ok {
+		return stringField(m, "displayName")
+	}
+	return ""
+}
+
+// InvoiceStatusTitle extracts status.title from an invoice row.
+func InvoiceStatusTitle(inv map[string]any) string {
+	if m, ok := inv["status"].(map[string]any); ok {
+		return stringField(m, "title")
+	}
+	return ""
+}
+
+// FlattenInvoiceRow copies nested fields for table output.
+func FlattenInvoiceRow(inv map[string]any) map[string]any {
+	out := map[string]any{}
+	for k, v := range inv {
+		out[k] = v
+	}
+	out["contactName"] = InvoiceContactName(inv)
+	out["statusTitle"] = InvoiceStatusTitle(inv)
+	if cur, ok := inv["currency"].(map[string]any); ok {
+		out["currency"] = stringField(cur, "abbreviation")
+	}
+	return out
+}
+
+// FindInvoiceItemByTitle returns the first catalog item with an exact title match.
+func FindInvoiceItemByTitle(items []map[string]any, title string) map[string]any {
+	want := strings.TrimSpace(title)
+	for _, it := range items {
+		if strings.TrimSpace(stringField(it, "title")) == want {
+			return it
+		}
+	}
+	return nil
+}
diff --git a/mails.go b/mails.go
index ac73997..5689c6c 100644
--- a/mails.go
+++ b/mails.go
@@ -5,6 +5,7 @@ package onlyoffice
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"net/mail"
 	"net/url"
@@ -96,11 +97,157 @@ func (c *Client) GetMailMessage(ctx context.Context, messageID string) (map[stri
 }
 
 // RemoveMailMessages deletes messages by id (PUT /api/2.0/mail/messages/remove).
+// The API response "response" field may be a number or object; success is HTTP 2xx.
 func (c *Client) RemoveMailMessages(ctx context.Context, ids ...int) (map[string]any, error) {
 	if len(ids) == 0 {
 		return nil, fmt.Errorf("RemoveMailMessages: at least one id is required")
 	}
-	return c.putJSONObject(ctx, "/api/2.0/mail/messages/remove", map[string]any{"ids": ids})
+	raw, err := c.putJSON(ctx, "/api/2.0/mail/messages/remove", map[string]any{"ids": ids})
+	if err != nil {
+		return nil, err
+	}
+	out := map[string]any{"ok": true, "ids": ids}
+	if resp, err := responseField(raw, "response"); err == nil {
+		var n json.Number
+		if json.Unmarshal(resp, &n) == nil {
+			out["response"] = n.String()
+		} else {
+			var m map[string]any
+			if json.Unmarshal(resp, &m) == nil {
+				out["response"] = m
+			} else {
+				out["response"] = string(resp)
+			}
+		}
+	}
+	return out, nil
+}
+
+// SaveMailDraftParams creates or updates a draft via PUT /api/2.0/mail/drafts/save.
+// Id 0 creates a new draft. Body is HTML; the API field name is "body" (not htmlBody).
+type SaveMailDraftParams struct {
+	ID      int64  // 0 = create
+	From    string // mailbox address, e.g. eslider@gmail.com
+	To      string // comma-separated or single address
+	Cc      string
+	Bcc     string
+	Subject string
+	Body    string // HTML
+}
+
+// SaveMailDraft saves a draft message. Returns the saved message map (incl. id).
+func (c *Client) SaveMailDraft(ctx context.Context, p SaveMailDraftParams) (map[string]any, error) {
+	if strings.TrimSpace(p.To) == "" {
+		return nil, fmt.Errorf("SaveMailDraft: to is required")
+	}
+	if strings.TrimSpace(p.From) == "" {
+		from, err := c.defaultMailFrom(ctx)
+		if err != nil {
+			return nil, err
+		}
+		p.From = from
+	}
+	body := map[string]any{
+		"id":      p.ID,
+		"from":    p.From,
+		"to":      p.To,
+		"cc":      p.Cc,
+		"bcc":     p.Bcc,
+		"subject": p.Subject,
+		"body":    p.Body,
+	}
+	return c.putJSONObject(ctx, "/api/2.0/mail/drafts/save", body)
+}
+
+func (c *Client) defaultMailFrom(ctx context.Context) (string, error) {
+	accounts, err := c.ListMailAccounts(ctx)
+	if err != nil {
+		return "", err
+	}
+	var fallback string
+	for _, a := range accounts {
+		email := strings.TrimSpace(stringFromMap(a, "email"))
+		if email == "" || email == "<nil>" {
+			continue
+		}
+		if boolField(a, "enabled") || boolField(a, "isDefault") {
+			return email, nil
+		}
+		if fallback == "" {
+			fallback = email
+		}
+	}
+	if fallback == "" {
+		return "", fmt.Errorf("SaveMailDraft: from is required (no mailbox on account)")
+	}
+	return fallback, nil
+}
+
+// AttachMailDocument attaches an OnlyOffice Files document to a mail message/draft.
+// fileID is the Documents/CRM file id (e.g. from InvoicePDFFile).
+func (c *Client) AttachMailDocument(ctx context.Context, messageID string, fileID int64) (map[string]any, error) {
+	id := strings.TrimSpace(messageID)
+	if id == "" {
+		return nil, fmt.Errorf("AttachMailDocument: message id is required")
+	}
+	if fileID <= 0 {
+		return nil, fmt.Errorf("AttachMailDocument: file id is required")
+	}
+	fields := url.Values{}
+	fields.Set("fileId", strconv.FormatInt(fileID, 10))
+	return c.postFormObject(ctx, fmt.Sprintf("/api/2.0/mail/messages/%s/document", url.PathEscape(id)), fields)
+}
+
+// PlainTextToMailHTML turns plain text into simple HTML paragraphs for drafts.
+// If s already looks like HTML, it is returned unchanged.
+func PlainTextToMailHTML(s string) string {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return ""
+	}
+	if strings.Contains(s, "<") && strings.Contains(s, ">") {
+		return s
+	}
+	parts := strings.Split(s, "\n\n")
+	var b strings.Builder
+	for _, p := range parts {
+		p = strings.TrimSpace(p)
+		if p == "" {
+			continue
+		}
+		b.WriteString("<p>")
+		b.WriteString(strings.ReplaceAll(p, "\n", "<br/>"))
+		b.WriteString("</p>\n")
+	}
+	return b.String()
+}
+
+// MailHTMLWithBlankParagraphs inserts empty paragraphs between blocks
+// (German Geschäftsbrief-style vertical spacing in OnlyOffice Mail).
+func MailHTMLWithBlankParagraphs(html string) string {
+	html = strings.TrimSpace(html)
+	if html == "" {
+		return ""
+	}
+	return strings.ReplaceAll(html, "</p>\n<p>", "</p>\n<p>&nbsp;</p>\n<p>")
+}
+
+// Int64FromMap coerces OnlyOffice numeric id fields (float64/int/string) to int64.
+func Int64FromMap(m map[string]any, key string) int64 {
+	switch v := m[key].(type) {
+	case float64:
+		return int64(v)
+	case int64:
+		return v
+	case int:
+		return int64(v)
+	case string:
+		n, _ := strconv.ParseInt(strings.TrimSpace(v), 10, 64)
+		return n
+	default:
+		n, _ := strconv.ParseInt(fmt.Sprint(m[key]), 10, 64)
+		return n
+	}
 }
 
 // ResolveMailFolder maps a CLI folder name or numeric string to a folder id.
diff --git a/mails_test.go b/mails_test.go
index 3302f0c..be81255 100644
--- a/mails_test.go
+++ b/mails_test.go
@@ -1,6 +1,9 @@
 package onlyoffice
 
-import "testing"
+import (
+	"strings"
+	"testing"
+)
 
 func TestResolveMailFolder(t *testing.T) {
 	tests := []struct {
@@ -70,3 +73,27 @@ func TestMailMessagesAsTableRows(t *testing.T) {
 		t.Fatalf("got %+v", rows[0])
 	}
 }
+
+func TestPlainTextToMailHTML(t *testing.T) {
+	got := PlainTextToMailHTML("Hello\n\nWorld\nline2")
+	if !strings.Contains(got, "<p>Hello</p>") || !strings.Contains(got, "World<br/>line2") {
+		t.Fatalf("got %q", got)
+	}
+	html := "<p>Already</p>"
+	if PlainTextToMailHTML(html) != html {
+		t.Fatalf("html passthrough failed")
+	}
+	spaced := MailHTMLWithBlankParagraphs("<p>A</p>\n<p>B</p>")
+	if spaced != "<p>A</p>\n<p>&nbsp;</p>\n<p>B</p>" {
+		t.Fatalf("spaced: %q", spaced)
+	}
+}
+
+func TestInt64FromMap(t *testing.T) {
+	if Int64FromMap(map[string]any{"id": float64(99)}, "id") != 99 {
+		t.Fatal("float64")
+	}
+	if Int64FromMap(map[string]any{"id": "42"}, "id") != 42 {
+		t.Fatal("string")
+	}
+}