I haven't had many problems w USPS or UPS, I use both depending on the size of the package and where it's going. If it's small enough to fit in a Priority Mail box and within the USA, USPS is a no-brainer. Cheap w tracking and fast.
I use UPS for larger boxes within the US, usually stuff I sell on eBay. I've never really had any problems w them, decent tracking and speed. There have been a couple times when packages get stuck in their system but they always popped-out on the other side if a little late.
For International shipping, it's the Post Office all the way. I recently sold a 964 air box to a guy in Aus. on this forum. I brought it first to UPS and it was outrageously expensive, maybe $400?
I guess they aren't interested in that facet of the business.
I next brought it to the Post Office and it was something like fifty dollars w fairly fast service. The customs form, as someone else pointed out, are required by governments, not the PO.
Lastly, I drop into the two Post Offices near me as well as others around town occasionally and none are bad in terms of service or wait unless you really go at a bad time.