Last time i used USPS it was very very difficult to just ship a package. The little kiosk only prints out a label that i still have to fill out and then they are generally rude as hell.
I love UPS just for the easiness of business. I go in, take whatever it is, weigh it and they slap a sticker on it. Yes, its a little more but at least you don't have to pay extra for a tracking number.
UPS is not a good choice for cross border shipping. They have their own little customs brokerage racket going on.