Hi Elana,
I've done things like this too. God, I'm the worst.
When I think of you bringing him to the front of your mind once a month, just long enough to send him another bill, I just tense up inside.
He isn't going to pay you.
He isn't learning a lesson by receiving your bills. He's probably burning YOURS without opening them.
You're not getting anything out of it but stress and contact with him (or the thought of him)
He doesn't deserve to be in your brain any more.
Your choice, naturally, but...you might think about whether this is doing you any good. A debt is a debt, but unless you REALLY want to pursue it through the court system, you may want to just send an annual invoice, right before a cleansing ritual (preferably one that involves burning him in effigy). Or better yet stop altogether and kick him out of your head.
He's not getting away with anything that he hasn't already gotten away with. Life will catch up with him in its proper time.
Hugs to you, I've been there.