My mailman always leaves things that are postage due. He leaves an envelope for me to put money in for him the next day. Of course, we are a very small town so maybe that isn't standard every where.
Oddly, when I took mine to the post office, the exact same card with the exact same squares in them did not cost the same to mail. I thought it was because of where they were going but the post master said it was because of thickness. Any regular envelope that does not slide "easily" through a certain slot takes more postage. I guess I squished some better than others!