Here is a fun one on a cover I saved 20 years ago just for the stamps.

It was mailed with just the First Class domestic rate of 29¢. It was then returned to the writer for an additional 21¢ in postage to make up the 50¢ first class rate for international destinations (in this case, Panama).



I am not sure why they so firmly scratched out the marking (although the needed postage was affixed).

Here is what it says:



I love the 2 slogan cancels juxtaposed as they are, and the early Youngstown, OH spray cancel is doubled because of course it was mailed twice


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