API introduction - Sendpaper

https://sendmypaper.com/v1

No API key. Create an order, send the person to its checkout_url, and it is mailed once they pay.

Idempotency

Send an Idempotency-Key header (or an idempotency_key field) on create calls. A repeat with the same key returns the original order with status 200 instead of 201, so retries never double-mail.

Errors

Errors share one shape. Validation errors use status 422 and list every bad field so an agent can fix them in one pass:

{
  "error": {
    "type": "invalid_request",
    "message": "Some fields are invalid.",
    "fields": [
      { "field": "to.zip", "message": "Use a 5-digit US ZIP code (or ZIP+4)" },
      { "field": "content", "message": "Give the front either an image URL or a headline" }
    ]
  }
}

Status	When
`404`	No such order
`409`	Order can’t be cancelled (already paid or closed)
`422`	Invalid request; see `fields`
`500`	Our side. Retry with the same idempotency key.