USPS has been my go to for most things. Prices have been most of the time lower with their flat rate boxes vs UPS/FedEx. Only exception is UPS/FedEx is cheaper on heavy, your own packaging boxes.
I recommend using PirateShip.com. If you ship Priority Mail, they discount that surcharge that USPS is charging for the season. They also offer good prices on first class padded packaging (your own packaging). However first class I believe they do not discount the surcharge.
I have a buddy in the shipping business and even he says their company has a hard time to compete against PirateShip.