Since I am an avid stalker of things paper related, I learned early on in wedding planning that a pen with black-light ink is a helpful tool for numbering RSVP cards. (Many people tread this path before me.)
And why is numbering RSVP cards helpful? It helps solve the puzzle of who replied if a guest forgets to write their name on the card. So if you keep a spreadsheet where each invitation has a number and you write those numbers on the corresponding RSVP card, you can track down any guest’s response.
This can be done with any kind of pen, but with normal pens the numbers are visible. That’s what I like about the black-light ink pen. You don’t see the number with the naked eye and maintain the card’s pristine look. When you shine the black light on the area you wrote on, the writing will appear again. You can get one for less than a dollar (sans shipping) on Amazon.
Are you numbering your RSVP cards? What kind of pen do you plan to use?