FWIW - with stamps.com, I can print postage and customs forms from home. You are supposed to hand it to a USPS employee, so usually I just hand it directly to our usual carrier as he delivers the rest of the mail - no need to go the PO.
One thing I have learned, however, is that you can't track flat rate boxes once they leave the US. To track them, you have to use the other Priority services. I've had a few anxious days waiting for a buyer to let me know it arrived, as I was flying blind overseas.