MountainGoth,
I have worked for both UPS and FedEx. I have since been very familiar with USPS shipping rates. Here's my advice for you. Yes, any small items will be the cheapest with the USPS, however, if you are shipping anything bigger than a mail bubbler, then you need to go with any of the "parcel" companies. Between UPS and FeddEx, the differences are usually:
FedEx is moving ground shipments faster than UPS, and is significantly cheaper.
Here are the rates I compared last Thursday when shipping a 11x8.5x6 in. I insure all of my shipments, this is with insurance included, all rates are rounded up.
-USPS- $30
-UPS- $27
-FedEx Ground- $14
Let me know if I can be of any more help!