I will be participating in a mail art project with Mark Philip Venema next month.
The rules state that each card must be unique (so not all 30 cards the same) and must be sent through the post (so not in an envelope) so that the handling process is visible on the work and perhaps even part of the concept, and all 30 made and posted in a one-month period.
I can’t wait!
At this writing there looks like there is still room in the group.
The limit is 30. (Then I think another group will form of the next 30.)