Oh man, oh man. Don't get me started on the utter mess that the UPS and FedEx APIs are. I've gone through the app certification process for both, as well as Stamps.com (USPS), and I think the only other project that rivals that mess is ADP (development took over a year, certification took another 6 months and 10s of thousands in fees.)

As an example, the developer document solely for obtaining shipment rates is 98 pages long.