PayPal gives you the commercial rate for shipping and doesn't charge any fee for the service. You are not limited to items shipped as a result of an ebay auction. You can ship anything to anyone. The same adhesive backed labels that work on the USPS site work win my printer on PayPal.

I thought the USPS site would be the answer several years ago, but I have nothing but trouble with their site interface. I can't even seem to be able to buy stamps on the USPS site!