I VERY much like PirateShip for all the reasons you and other have mentioned.

I have even sent media mail and larger boxes. I have generated postage for items to be sent TO me from others. Then sent them the PDF, they print and mail.

A couple of times the postage was too much or not enough (the people gave me the wrong weight). PirateShip charged me a little more or gave me a refund. Better than have it returned with postage due.