I have hundreds of them that are cancelled on the first day of issue of a new stamp, at the post office where they were first sold. Nobody is interested any more. These go back to the 1940s and 50s.

If you got tired of sending cash and addressed envelopes to the postmaster in Podunk,AL, you could pay a company to imprint an appropriate design on an envelope and get them postmarked for you (cachet covers). Artmaster and ArtCraft were the two majors.