As long as it's a flat rate box, you can ship up the max the P.O allows weight wise (70lb) and not pay more than the flat rate box cost. (I say it like that as there a few different rate boxes.) I am GUESSING he is trying to get $ back out of you b/c you charged him more than exact shipping. Just guessing here.
Also, def. call the P.O. If they had a postage due on him for that amount, they should have record of it. I don't know if they can tell you that, but definitely check and talk to the accountables clerk, explain the situation and ask if they can at least tell you if they have a record of postage due on that package from you.