SendLetter

eu.sendletterPublished by sendletter.euOfficial publisherv1.0.0Updated Oct 24, 2026

Send real, printed letters by post across Europe. Requires a free account; quote before sending.

VerifiedStreamable HTTPWeb executableProductivity & WorkflowBusiness & Commerce

Overview

AI-generated overview

SendLetter lets an assistant prepare, quote and send real printed letters by post across Europe, then track their status and invoices.

What you can do
Prepare a letter from text, rich text, a base64 file or a file URL, and get back a PDF preview and exact total before sending it, with standard, priority or registered delivery and optional colour printing. Check a recipient address and get a price quote for a destination and page count before spending. Send, list, fetch, cancel and download letters, including proof of posting. List and download invoices and credit notes. Receive status-change webhooks covering submission, printing, posting, delivery, failure and refund.
When to use it
Useful when an assistant should turn a document or message into a physical letter mailed within Europe, for example invoice reminders or formal correspondence, with a preview and price shown for approval first. Test keys let you build and verify the whole flow without printing or charging.
Requirements
A remote endpoint at sendletter.eu with OAuth sign-in, and a free SendLetter account. Sending needs an API key, which the README shows passed as SENDLETTER_API_KEY; webhook verification needs a signing secret, shown as SENDLETTER_WEBHOOK_SECRET. Keys starting sk_test_ run in test mode.
Before you install
Live letters cost money and draw on a wallet balance, so confirm the PDF and exact total with the user before sending. Sending is irreversible once a letter is posted, so pass an idempotency key on anything retryable. Supply content exactly one way. Verify webhook signatures against the raw body and deduplicate on event id, or a repeated event could bill a customer twice. Sender and recipient postal addresses are sent to SendLetter.

Installation

In SourceWeft

  1. Open SendLetter in the dashboard and add it to a workspace.
  2. Connect the required credentials. They are stored privately and never shown on public pages.
  3. Enable the server for the chats that should use its tools.

Web executable via Streamable HTTP. Remote servers run from the web runtime once configured in a workspace.

Use with an agent

Paste this prompt into Claude Code, Codex, Cursor or another agent — or into SourceWeft chat. The agent reads this server's install guide, shows you its publisher, endpoint and sign-in, and adds it to your client once you agree. Keys never go into a command.

Read https://sourceweft.com/mcp/eu.sendletter%2Fsendletter/install.md and connect the MCP server it describes to my client. Before changing any config, show me its publisher, endpoint and sign-in method and wait for my OK. Never put an API key in a command or in a file I would commit; ask me where it should come from.

Read the install guide the agent follows

Other MCP clients

Add this to your client's mcpServers config. The client completes OAuth sign-in on first connect.

{
  "mcpServers": {
    "sendletter": {
      "type": "http",
      "url": "https://sendletter.eu/api/mcp"
    }
  }
}

README

sendletter

Send a real letter — printed, folded, franked and handed to the post — from Node.

bash
npm install sendletter
ts
import { SendLetter } from 'sendletter'
const sendletter = new SendLetter(process.env.SENDLETTER_API_KEY!)
const input = {  idempotencyKey: 'invoice-2026-0042-reminder-1',  sender: {    company: 'Twilper',    name: 'Gert Snijder',    street: 'Merelstraat',    number: '64',    postalCode: '8916 AX',    city: 'Leeuwarden',    country: 'NL',  },  recipient: {    name: 'Jan de Vries',    street: 'Keizersgracht',    number: '123',    postalCode: '1015 CJ',    city: 'Amsterdam',    country: 'NL',  },  text: 'Beste heer De Vries,\n\nBijgaand de herinnering voor factuur 2026-0042.',}
const prepared = await sendletter.prepare(input)console.log(prepared.id, prepared.pages, prepared.totalCents)const pdf = await sendletter.download(prepared.id)

Show the PDF and exact total to the user. After approval, use the same input and key:

ts
const letter = await sendletter.send({ ...input, expectedTotalCents: prepared.totalCents })
console.log(letter.id, letter.status, letter.totalCents)

Get a key at sendletter.eu. Keys starting sk_test_ post nothing, charge nothing, and still run the whole status chain, so you can build against the real thing.

Sending

Give the content exactly one way: text, document (rich text), file (base64) or fileUrl. Two is refused rather than guessed at, because the wrong document in a postbox cannot be recalled.

ts
// An invoice you already have as a PDF.await sendletter.send({  sender,  recipient,  file: { name: 'invoice.pdf', contentBase64: pdf.toString('base64') },  product: 'registered',        // 'standard' | 'priority' | 'registered'  colour: true,  idempotencyKey: `invoice-${invoice.id}`,})

Send an idempotencyKey on anything that can be retried. A repeat with the same key returns the original letter instead of a second envelope. That single field is what makes a retry after a network timeout safe, and a timeout says nothing about whether the letter was accepted.

Reading

ts
await sendletter.get(id)                       // one letterawait sendletter.list({ status: 'posted' })    // one pageawait sendletter.cancel(id, 'order withdrawn') // while it is still cancellableawait sendletter.download(id)                  // the PDF as printedawait sendletter.download(id, { proof: true }) // proof of posting
for await (const letter of sendletter.all({ mode: 'live' })) {  // walks every page; you never hold the cursor}

Invoices and credit notes

Every paid live letter receives its invoice number at payment time. Test letters never consume the statutory series. A full refund keeps that invoice and adds a separately numbered negative credit note.

ts
const page = await sendletter.listInvoices()await writeFile('invoice.pdf', await sendletter.downloadInvoice(page.data[0].id))
if (page.data[0].creditNote) {  await writeFile(    'credit-note.pdf',    await sendletter.downloadCreditNote(page.data[0].creditNote.id),  )}

Checking before you spend

ts
const check = await sendletter.validateAddress(recipient)if (!check.valid) console.log(check.problems)   // answers, does not throw
const quote = await sendletter.quote({ destination: 'DE', pages: 3 })

validateAddress returns a bad address rather than throwing: a wrong postcode is the successful outcome of asking. supported: false is the one that cannot be fixed by editing the address — we do not carry to that country.

Errors

Everything the API refuses becomes a SendLetterError with a code to branch on.

ts
import { SendLetterError } from 'sendletter'
try {  await sendletter.send({ sender, recipient, text })} catch (error) {  if (!(error instanceof SendLetterError)) throw error
  if (error.code === 'insufficient_balance') {    // Put this in front of the customer; the wallet is short, nothing else.    return redirect(error.topUpUrl!)  }  if (error.isRetryable) {    await sleep((error.retryAfter ?? 5) * 1000)  }}

isRetryable covers 429 and 5xx. Nothing else should be retried: a 400 means the letter will be refused just as firmly the second time.

Webhooks

Status changes arrive as a POST to the URL you registered. Verify them. Without that, anyone who learns your endpoint can tell your system a letter was delivered.

ts
import { verifyWebhook, SendLetterError } from 'sendletter'
export async function POST(request: Request) {  const rawBody = await request.text()   // the raw text, not the parsed object
  let event  try {    event = verifyWebhook({      rawBody,      signature: request.headers.get('x-sendletter-signature'),      secret: process.env.SENDLETTER_WEBHOOK_SECRET!,    })  } catch (error) {    if (error instanceof SendLetterError) return new Response('nope', { status: 400 })    throw error  }
  if (await alreadyHandled(event.id)) return new Response('ok')   // see below  await handle(event)                                            // letter.posted, ...  return new Response('ok')}

Two things this shape gets right and hand-rolled verification usually does not:

  • The raw body, not a re-serialised one. The signature covers the exact bytes we sent, and JSON.stringify does not promise to reproduce them. Read the body as text, verify, then parse.
  • Deduplicate on event.id. Delivery is at-least-once with retries, so a timeout on your side means the same event arrives again. letter.posted handled twice should not bill a customer twice.

Events: letter.submitted, letter.printed, letter.posted, letter.delivered, letter.failed, letter.refunded.

Test mode

A sk_test_ key runs the full chain — submitted → printed → posted → delivered — with webhooks firing exactly as they will in production, while touching no wallet and no printer. client.isTestMode tells you which kind of key you are holding, which is worth asserting in a deploy check: the two look alike in a log and only one of them costs money.

Reference

sendletter.eu/en/developers · OpenAPI at /api/v1/openapi.json · MIT

Source: README.md at commit fca2556

Tools

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Version history

1
  1. v1.0.0LatestSep 16, 2026

Score

85% of the applicable checks met

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