I think it´d be very pretty to have a tarot stamp set! Even for non-believers like myself, it´s difficult to deny tarot cards are pretty. It can also become a pretty long collection considering there are major and minor arcana. They could also be unlocked instead of bought (maybe by doing special things on the app, like an easter egg), and the possibility of sending them inverted could be bought.

For example, The World stamp could be given to individuals who have sent a letter to people of all countries with the "around the world" stamp. Or, another example, The Hierophant could be unlocked if you´ve spoken to a certain number of people interested in religion. Maybe Death could be obtained by sending a letter to someone during early morning/late night? Anyway, I think it´d be cool.