According to Seinfeld episode 908 (The Betrayal), an unvitation (sometimes spelled un-vitation) is when you send someone an invitation to an event knowing that the person will not attend. Unvitation are usually sent for events in which the invitees give gifts to the inviter (a birthday or wedding for example). The hope is that the people who can’t attend will feel guilty and respond by sending nicer presents.
Billy: How come you get better birthday gifts from your Grandmother then I do?
Bobby: Because my Grandmother lives 3,000 miles away and I always send her an unvitation to my birthday party.