I had that same trouble here- there seems to be an international union of post offices. I had different card sizes: some were too big and some were too small- but only by a very small space. Now the postage for US is very expensive here anyway: It's 1,65 Euro which is more than 1,80 US$. And then I should have 3 Euro on each card- on the smaller ones as well as the bigger ones.

Well I put the 1,65 postage on each and put them in a mailbox. None of the smaller ones came back and only a few of the bigger ones. I put the additional postage on the ones that came back and off they went again.

I also learned that I would not have been allowed to re-use un-used stamps if I had put the small ones in bigger envelopes. This upset me especially as I had bought these extra cute cat stamps that are more expensive because they contain a charity sum.

So I cheated them by just putting them in the mailbox but although I am a very brave citizen who pays all his taxes I have no bad conscience at all.