Well, that depends on what you expect in terms of speed, insurance, pick-up service and so on.
I don't live in America, so take my statements with a huge grain of salt, but from what I've read over the years here and elsewhere, one of the cheaper options is still the good old USPS.
If you want express service like overnight delivery etc. it's probably better to use UPS or FedEx, but of course that's more expensive.
My impression is that most people in our M2M sales who ship within CONUS use in fact the cheapest USPS option there is with tracking, as it's probably the most cost effective. Also, if we're talking reps, you can't profit from the higher insurance coverage that you may get at FedEx etc. anyways.