I was going to give UPS a chance over USPS for doing business shipping. Printing labels through paypal went relatively smoothly though it was a bit more expensive than USPS.
While the shipping calculator was close for the most of the items being sold on eBay, a few of the charges came out really, really inaccurate.
About a month after I started using them, I got an invoice for a bunch of hidden charges that were never mentioned and that I should've been billed for up front when I originally bought the postage. This is the scummiest kind of bait and switch.