Wiktionary
prestamped
a. Having been stamped in advance.
Usage examples of "prestamped".
The next five finished their messages, sealed them with prestamped wax and saluted before dashing off towards the stables.
But we found the stack of prestamped mailing envelopes which the fiend was using to get rid of the stolen treasures, and these were quite thick and large.
A postcard of the Acropolis, prestamped envelopes, packaged towelettes bearing the insignia of Olympic Airlines.
Little did she know I was, by nature, so cheap that I'd sit there and pick the stamp off the prestamped envelope.