USPS APIs (REST, OAuth 2.0)

USPS APIs at developer.usps.com are the modern USPS surface for label creation, rate calculation, tracking, address validation, and pickup scheduling. They are REST-based with JSON payloads and use OAuth 2.0 client_credentials for authentication. They replace the legacy Web Tools XML services and are the integration target USPS is investing in. Authentication is a standard OAuth 2.0 client_credentials grant. Cache the bearer token centrally, refresh with a buffer before expiry, and consolidate concurrent refresh attempts so the token endpoint is not rate-limited during a token-storm. Many USPS APIs operations require both an authenticated OAuth call and a USPS account-tied identifier (CRID, MID, or permit number) provided in the request body — the OAuth credentials authenticate the integrator, while the identifier authorizes the specific shipping account. Operationally, USPS APIs return more structured error detail than Web Tools and provide better-shaped problem responses for invalid addresses, unsupported services, and rate disqualifications. Tracking events are eventually consistent. Webhooks for tracking are available through a separate USPS Informed Visibility product, not through the core developer portal. Sandbox returns deterministic test data and is the right environment for contract testing; rate accuracy and account-tied entitlements should be verified in production with a small parallel-running window.