not a v-day card per se but relevant.
that's a card that should be given by mail on a regular basis to someone you're already in a relationship with--preferably the same card that you take from them after they've begin to lose interest in it - not out of malice or anything, just the novelty wearing off - and then remail it, making clear that it is the same card by, I dunno, signing your name AGAIN. Hopefully this will at some point inspire them to mail it to you.
This card is a Spark Notes guide to an enduring relationship, provided you both keep interest in each other.
Fidelity through a joke strikes me as a bit romantic~