Docs

Ship mail from your stack.

Start in the dashboard, then graduate to API keys and MCP. Demo mode uses a local store so every path is exercisable without Firebase or LetterStream.

REST API

OpenAPI 3.1, Bearer lbl_ keys, PDF upload, letters, pricing, and address validation.

MCP server

Tools for Claude Desktop and Cursor: send_letter, status, jobs, pricing.

x402 + USDC

HTTP 402, PAYMENT-REQUIRED, PAYMENT-SIGNATURE, Base and Solana exact scheme.

Fonts

Uploaded PDFs keep embedded fonts. Compose uses Times, Helvetica, and Courier.

Product notes

Create an account (or enter the demo workspace), add a from-address, compose a letter, and pay with credits. Top up with Square (cards) or USDC via x402.

Live LetterStream activation happens after approval. Set LETTERSTREAM_API_ID and LETTERSTREAM_API_KEY, then set LETTERSBYLETTER_MODE=live (or LETTERSTREAM_LIVE=true). Without credentials the app stays in demo and never invents a successful live send. Paying to send uploads a ZIP with preauth=1 and authorizes only on that confirmed submit. Price checks never release mail. Optional LETTERSTREAM_DRY_RUN=true builds the ZIP without calling LetterStream.