I have done several of these swaps and like I said, the people at the PO are looney tunes. I have gotten some with postage due that I could not figure out why. I just pay it because I will get another envelope in the same mailing looks the same and no postage due. I blame the PO not the person who sends it. I am just happy to get it. I don't think there is any way to truly figure it out so I quit trying.