https://developer.stamps.com/
USPS Shipping API, Developer Program - Stamps.com
May 11, 2021 - Integrate with the Stamps.com Developer Program to access the number 1 USPS Shipping API. Our REST API lets you access USPS rates, print shipping labels and...
api developer programusps shippingstamps
https://developer.paychex.com/
Paychex Developer Program API Developer Center
developer programpaychexapicenter