UPS is the most expensive, but they are the best. Good online tracking, good website.
I've never used FedEx because they don't have an office where I live.
U.S. Post Office is the least expensive, but they're the least reliable (they lost a set of OEM tails I sold few years back) and not as fast. USPS flat rate boxes are a good deal. No matter how much something weighs or how far it's going, it costs the same as long as it fits in the box. They have different sizes - the smaller the cheaper of course. Go to the Post Office to get some Flat Rate boxes, you can get as many as you want, they're all free - part of the reason the Post Office runs a defecit every year, heh. But don't count on USPS tracking numbers - more than half the time you enter them into the website they don't work, or they finally do work the day the package shows up at your house so what's the freakin point. The website sucks - a government website - hmmmm. Yeah, please run my healthcare, Obama.