For a long time the Post Office has issued stamps with a big letter as opposed to the number of cents of postage, the idea was to have a temporary first class stamp ready to sell before the rate increase was decided on.
No problem except that they did something stupid. They had both a letter G first class stamp and a letter G postcard stamp. The latter costs less and had "postcard rate" in tiny oprint added, too small for me to read without bifocals.
So one day I was looking around for postcard stamps and "couldn't find any" around the house. I made a special gas wasting trip to the post office to buy some.
Then I found the sheet of G postcard stamps which i remembered using about a dozen oif thos sstamps to mail ordijnary letters.
None of them came back for postage due. Perhaps the Post Office stamp canceling machinery couldn't tell the difference either.
Disney hints:
http://members.aol.com/ajaynejr/disney.htm
By the way, even if it was my mistake, if I got back one of my letters for postage due, I just add the missing amount and remail it, reusing the first stamp.
If you want to make your own stamps with your face or some favorite object or pet pictured, go ahead and do so, but put that stamp on the back like an Easter Seal. One of these days the Post Office might set up portrait photo booths so you can make real stamps (at a higher price) with your picture on them.