API Reference

Send physical letters anywhere in Canada programmatically. One endpoint, three modes.

Quick Start

Send a letter in one API call. You need an account with a payment method and an API key.

1. Create an account at sendletter.app/signup
2. Add a payment method on your profile
3. Generate an API key on your profile
4. Send a POST request:

curl -X POST https://sendletter.app/api/v1/send \
  -H "Authorization: Bearer sl_live_YOUR_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "mode": "draft",
    "letter_size": "standard",
    "from": {
      "name": "Jane Smith",
      "line1": "123 Maple St",
      "city": "Toronto",
      "province": "ON",
      "postal_code": "M5V 2T6",
      "country": "CA"
    },
    "to": {
      "name": "John Doe",
      "line1": "456 Oak Ave",
      "city": "Vancouver",
      "province": "BC",
      "postal_code": "V6B 1A1",
      "country": "CA"
    },
    "letter": {
      "date": "2026-03-23",
      "salutation": "Dear John,",
      "body": "This is a test letter sent via the sendletter API.",
      "closing": "Sincerely,",
      "signature": "Jane Smith"
    }
  }'

Authentication

All requests require a Bearer token in the Authorization header.

Authorization: Bearer sl_live_YOUR_API_KEY

API keys start with sl_live_. Keep your key secret. If compromised, regenerate it from your profile — this immediately invalidates the old key.

Your account must have a valid payment method on file. Requests without one return 402.

POST /api/v1/send

Creates a letter order. The letter is queued for printing and mailed within 1 business day via Canada Post.

Request Body

Response

{
  "id": "uuid-of-order",
  "status": "queued",
  "letter_size": "standard",
  "amount_cents": 379
}

Modes

Draft Mode

Provide structured letter fields. We render them into a properly formatted letter page.

Formatted Mode

Send your own HTML content. We wrap it in a print-safe page template with proper dimensions (8.5×11" or 8.5×14" at 72dpi, 1" margins). Max 500 KB.

{
  "mode": "formatted",
  "html": "<h1>Hello World</h1><p>Custom HTML letter content.</p>",
  "css": "h1 { color: navy; }",
  "font": "Georgia",
  "font_size": 14
}

Upload Mode

Upload a pre-formatted PDF or DOCX file as a base64 string. Max 10 MB, max 15 pages.

{
  "mode": "upload",
  "file": "JVBERi0xLjQK...",
  "file_type": "pdf",
  "page_count": 2
}

Addresses

Options

Letter Sizes

Fonts

Available for draft and formatted modes:

• Times New Roman (default)
• Georgia
• Arial
• Helvetica
• Courier New
• Verdana

Pricing

All prices are in Canadian dollars (CAD) and include printing, envelope, Canada Post postage, and tax.

Error Codes

All error responses include an error field with a human-readable message:

{ "error": "to.postal_code is not a valid Canadian postal code. Expected format: A1A 1A1" }

Billing

API usage is billed daily. Each letter is charged at the listed price. Charges are processed automatically against your saved payment method. You can view usage and billing history on your profile.

Rate Limits & Constraints