postalmuseWhat is supported.
US postcards and US Letter documents. Free API testing is available to everyone.
| Live sending | Enabled |
| Postcards | 4×6 or 6×9 · text, photo, or artwork |
| Letters | PDF or text · 20 source pages |
| Private uploads | PDF, PNG, JPEG · 10 MB |
| International / Certified / scheduling | Not enabled |
Machine-readable capabilities →
Sandbox address checks are simulated. Provider postal processing may update addresses; exact-destination guarantees are unavailable.