The project that I am currently working on requires that I a calculate shipping for international addresses. This is pretty straight-forward for UPS, but that can get kind of pricy, so I have been integrating the USPS rates api from the United States Postal Service.
Let me just say that the USPS is a pain. The array of options is dizzying and getting something back useful from the api is pretty hard.
It is working, but it sure feels dirty.