Other Office Pens & Pencils for sale | eBay
I think it's absurd that this is potentially necessary, but it could be useful when people are too rude/stupid to put their names on an RSVP. It's a invisible ink pen, write the guests name on the RSVP card before you send it out. If it comes back without a name on it, you will know who it is from. This has saved me A LOT of headaches.