Some of you might be old enough to remember the days when the post office sold Airmail envelopes/paper
They were like an A3 sheet (imperial equivalent) and were like tissue paper (to keep the weight down). You wrote in the space available and folded along the indicated lines almost like tuck tab A in pocket B type of thing They were Airmail Blue and preprinted with stickers
Ah? Those where the days
Anyway
Shouldn't Discworld have something similar for the fast Pony Express Post,
Letter from somebody for us to read and a nice Pony Post Stamp