Check out the Redlink API.
The Zip Code API provides the following functions: Distance Between Zip Code API - The API allows you to quickly and easily figure out the distance between zip codes. Zip Codes in Radius API - You can use this API to find all zip codes within a given radius from another zip code. Zip Code to City and State API and Zip Code to Latitude/Longitude API - The API can convert a zip code to the primary location for the zip code. This includes the city, state, latitude, and longitude for the primary location. City to Zip Code - This API looks up the Zip Code based on the City and State.
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