We got back an RSVP card and the guests are not able to attend the wedding but sent a check. They didn't post date the check and I refuse to cash it before the wedding (it will expire like 5 days before the wedding. . . at least if I'm correct in my thinking that checks are only good for 90 days). What do I say in the thank you card in this instance since they are going to know we didn't cash the check?