What does "opening an account" entail? I hadn't heard that before and I rarely use USPS.com for labels because I'd only been able to use the higher-dollar services.
As to queen_mab's problem, that's a weird one. If nothing else, though, try doing it all manually through this link:
http://www.paypal.com/us/cgi-bin/webscr?cmd=_ship-now
It'll make you log in but it should work.