An API for the mailbox

Let your agents send physical mail.

Your agent prepares the letter. You review the PDF and approve the cost. Then it can request a test submission.

Try the staging dashboard Read the API docs

US letters. Prepaid credit. Human approval for every letter.
Test outcomes are local simulation or a provider job held for authorization. No physical mail is sent.

DRAFT → REVIEW → SENDHUMAN APPROVAL REQUIRED

From text to letter

A checkpoint before
the mailbox.

Give your agent a named key with a spending cap. Keep approval in your own signed-in dashboard.

Your agent prepares

Send text, a US return address, and a recipient to the API. Get an immutable PDF, a provisional test-credit quote, and an approval link.

You check the details

Open the PDF. Review the addresses, wording, and quoted cost. For LetterStream tests, an additional address coversheet is added by the provider and is not in this preview.

Submit a test

After approval, you or your agent can queue the letter. Follow its status in the dashboard. The result is a local simulation or a LetterStream test job held for authorization.

A small, explicit API

A letter starts
with a request.

Prepare once. Review the result. Send after approval. Idempotency keys make retries safe without creating another letter.

API keys can prepare and send approved letters. Buying credit and approving a letter require a human session.

Follow the curl walkthrough

Illustrative staging response

{
  "id": "00000000-0000-4000-8000-000000000001",
  "status": "awaiting_approval",
  "mode": "letterstream_test",
  "service": "first_class",
  "page_count": 1,
  "quote": {
    "amount_cents": 349,
    "currency": "USD",
    "expires_at": "2026-10-01T13:00:00Z"
  },
  "approval_url": "/app/?letter=00000000-0000-4000-8000-000000000001"
}

Partial example. $3.49 is provisional test credit, not a live price or provider quote. Use the quote returned for your letter.

US letters, one at a time

Choose a mail service.

Both options run in Test Mode. Review the provisional test-credit quote before approving. Provider-reported cost is shown separately; it is not a live price offer.

FIRST_CLASS

First-Class

For ordinary correspondence to a US address. Prepare a letter from plain text and inspect the generated PDF.

Staging only · No physical dispatch

CERTIFIED_RETURN_RECEIPT

Certified + electronic return receipt

Exercise the certified-mail workflow in staging. Test results are not proof of mailing, receipt, or delivery.

Staging only · No real return receipt

Before you try it

Use sample letters.
Keep sensitive details out.

Staging is for testing the workflow. Use fictitious names, addresses, and letter text. Keep API keys in your agent's secret storage.

Will this send real mail or charge my card?

No physical mail is sent in staging. Checkout uses test payments, and any balance is test credit. Do not enter real card details.

What does the provider receive?

In LetterStream Test Mode, the synthetic PDF and addresses are submitted to the provider. LetterStream adds an address coversheet that is not in your preview, then holds the job for authorization. AgentPostage does not authorize physical dispatch. Local simulation does not submit to a provider.

Can an agent approve its own letter?

No. A signed-in human must review the letter and approve the exact quote. Approval itself does not send the letter.

Can I edit or cancel a letter?

Prepared content is immutable. To change it, prepare a new letter and review it again. Cancellation is available only before the sender claims the submission.

Can I use this for a legal deadline?

Staging provides no real mailing or delivery evidence. Do not rely on it for notices, deadlines, or legal effectiveness.