So if you’re a regular/often purchaser of Mack Weldon underwear (which I am), you can send your used clothes, including underwear, to them and they’ll send it along to donation centers.
I go on a “fetish” website about men’s underwear and have asked guys if they want some of my underwear. It’s usually stuff that doesn’t really fit well or I just don’t like it anymore or I want to make room for new stuff. They message me their addresses and I’ll package it up and send it off. Once I even sent a box to the UK! Sure, it costs me a few bucks, but it’s kind of exciting knowing that another guy is wearing my gear. Sometimes they’ll send a pic wearing them.
On that same site, I’ve actually met a buddy who I’ve given my underwear to a few times when he’s been visiting in my town. The last time we met up he asked me to model some of them for him. That was fun.