I have been using PayPal to print postage for awhile now. It is very easy. They do require that you get delivery confirmation on every order. No big deal to me, as I usually want it.
It is very easy to use with the Priority mailers. It is basically the same as going through the USPS site. You get a small savings by printing priority postage online, too.
You can also do this for UPS and FedEx as well.