I got a black light pen so MOH-Ho can track her RSVPs, in case any of them comes in illegible, damaged in the mail (it is a postcard, afterall), or my favorite - blank. I actually tracked my own by marking them with an inkless pen, but this is definitely safer and easier. I found this pen for barely over a buck (shipping included!), so I thought I'd show you the results and share the deal:
You really can't tell the markings at all under normal lighting:
I bought the pen HERE, and shipping took like 3 weeks (I was in no hurry so it didn't matter). Packaging was crappy (definitely don't send it directly as a gift), but the pen itself works perfectly. $1.33 for a good little spy pen to arrive right in my mailbox, really can't be beat.