You can get shipping quotes on USPS.com with the weight of your package and the zip codes.
If you pay online, you can save a few cents, print the lable at home and bypass the line at the Post Office and it there is no charge for a tracking #.
You can pay via Credit Card or from your PayPal balance.
I just printed a shipping lable through PayPal ( click on "print lable" in the transaction summary line).
If you accept PayPal as a "Gift", it is against the their terms of service and you also do not have any buyer protection ( no way to get your money back if the deal goes wrong).