Can I put stamps in the postage due envelope that the postman left in our mailbox?

Question

Our postman left an envelope in our mailbox that we're suppose to put money in for postage due on an over-sized postcard we received. Can I just put the equivalent amount in stamps in the envelope or does it have to be currency?

This is in the U.S.

Answer 1

It has to be money. Stamps are because of taxes, but if they need money because it is oversized then it has to be money.

Answer 2

when they leave that envelope, it's because the postal worker has used their own funds to make up the difference of the postage for you.  It would be nice if you left them the coinage.