Docs

Ship mail from your stack.

Production mailing is live via LetterStream. Start in the dashboard, then graduate to API keys and the hosted MCP endpoint. Staging keys (lbl_test_) never purchase postage. People sending one letter can follow the send guide.

REST API

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

MCP server

Guest x402 send_letter, org credits, Square card checkout, account bind, 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, add a from-address, compose a letter, and pay with credits. Top up with Square (cards) or USDC via x402. Live health (probed, not a static green light): /status and /status.json.

This production deployment has live LetterStream fulfillment. Staging API keys (lbl_test_) never purchase postage — that is a developer sandbox, not a postage-disabled site. A local checkout without LETTERSTREAM_API_ID and LETTERSTREAM_API_KEY stays in labeled 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.