Introduction

Updated 9/17/2026

LetterAPI

LetterAPI turns a physical letter into a single API call. Upload a file, hit an endpoint, and a print shop prints, folds, and posts it — pickup or Royal Mail, from £0.86 per letter.

This documentation is for developers integrating LetterAPI into an application or automated workflow. If you're looking for the marketing site, see letterapi.uk.

How it works

Create an API key from your dashboard (test or live mode).
Create an order — POST /api/v1/orders with a file reference, paper/finishing options, and (for postal orders) a recipient address.
Pay for the order — via your account credit balance, or a one-off Stripe payment.
LetterAPI routes the order to an approved print shop, which prints, finishes, and either holds it for pickup or posts it via Royal Mail.
You receive webhooks as the order moves through paid → printing → posted/ready_for_pickup.

Base URL

https://letterapi.uk/api/v1

All requests and responses are JSON.

Test mode vs live mode

Every API key is created in test or live mode (la_test_... / la_live_...). Orders created with a test key:

are flagged isTest: true
are paid instantly via a simulated payment intent — no real card or credit deduction
never leave your account or touch a real print shop queue in the way a live order does
deliver webhooks to your test webhook endpoint, independently of your live one

This lets you build and test your entire integration — order creation, payment, webhook handling — without spending money or printing anything. Switch to a live key when you're ready to send real mail.

Where to go next

Authentication — API keys and request signing
Creating orders — the order lifecycle and request/response shapes
Webhooks — subscribing to order status changes
Errors — error shapes and HTTP status codes

LetterAPIPrint and post letters in the UK, from one letter to thousands.