USPS Web Tools API
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A set of free APIs provided by the United States Postal Service for address validation, ZIP code lookup, rate calculation, and package tracking.
USPS Web Tools is a suite of RESTful APIs available to registered users at no cost. The most commonly used APIs include: Address Validation (standardize and verify mailing addresses), ZIPCode Lookup (find the ZIP code for an address), CityStateLookup (find the city and state for a ZIP code), and Package Tracking.
To use the APIs, developers register for a USPS Web Tools account and receive a User ID. Requests are sent as XML over HTTPS, and responses are returned in XML format. Daily request limits apply and vary by API.
The address validation API is particularly valuable because it queries the official USPS address database (AMS) and returns standardized addresses with ZIP+4 codes appended. This is the same database used internally by USPS sorting equipment.