You can use "Zones" API to lookup US domestic zone information. The information is provided and defined by USPS. Teapplix subscribes from USPS zone information and updates our database periodically.
You can do zone lookups in bulk, providing many pair of "From" "To" zip codes (5 digits). Notice currently, zone lookup is limited to domestic US only.
The zone lookup api "Zones" is provided at the below endpoint: