Stamp

Stamps were used on letters and postcards.

Jo Grant once used a Doctor Who stamp to send her space-time postcard to Mike Yates. (PROSE: The Dark Scrolls of the Valeyard)

On the postcards sent by the Doctor to their companions, there were stamps of: a space station, Elizabeth X, Tiaanamat and a Kerb!am Man. (PROSE: From the universe with love!)

The Thirteenth Doctor noted how Hakim Khan collected junk like stamps. (TV: Arachnids in the UK)