Hi Ariadne, I'm a "regular" in here who's been kinda busy this week, so I couldn't comment earlier. Welcome!

About the guitar thing:
If you have to find an approropriate box to mail it, pack the damn thing so it won't break, take it to the post office and pay for shipping costs, by my expert math skills that's at least four more hoops you just jumped through for this jerk.

I'd send him an email telling him he has one week to pick it up, you'll leave it on the porch, and after that, it's getting donated to a local shelter or childrens' home. (Choose somewhere specific.)

Then donate it. You really don't want it hanging around to remind you of him. Getting rid of it will also bring some closure to you: just think of it as getting rid of him.