So FI and I were talking about how it is oh so hard to check a box and send back a pre-addressed, pre-stamped RSVP and he came up with this gem: "You know, if I was oozing money, I would have sent all the envelopes certified return receipt and put a note in the invitation "A non-response constitutes as a decline; so if you show up without responding, please bring your own food and a chair." I like his idea, haha.