Usually we really send postcards with a stamp and writing on the back--it's part of the postcard ethos--but occasionally some people have sent small prints in envelopes, particularly for more delicate processes.

You're really going to print up 30 little carbon prints for this? If that's more than you had planned, you might consider doing the group print exchange (4-6 prints), the blind print exchange (1 print), or the Traveling Portfolio (1 print and you get it back) for the carbons, and just print up postcards for the postcard exchange, much as I'm sure everyone would be delighted to get a carbon print in the mail.