So you have been waiting far too long for your Amazon shipment to arrive and are confused about who is actually shipping the package, UPS or USPS. Not only is Amazon now using thier own slocal shipping service they also sometimes combine shipping via UPS and the United Postal Service. You can track these packages using the mini package search.
Another frustrating package tracking situation is Ebay's Global Shipping program where they use a UPAAA tracking code.
Here is how to track Ebay UPAAA package.