So after the 2nd fanartist to 'grace' my comments section on AO3 with an offer for commissioned fanart, I've written a general response
in a Dreamwidth post for future offers. If you run into the same situation feel free to point the person to that post.
I have to admit, I'm a fandom old. Or more accurately, a fandom ooooold. Fandom at the time I started writing fic (the 1980s) was very careful about not pissing off the creators of their source material. If you charged much more than what it took to make your fandom stuff the community kind of kicked you out. (A small amount of profit was allowed; nobody was going to kick you out for charging an extra 20 cents.)
Nowadays, fanfic and other fanworks are more mainstream and creators are often more flexible about fanworks. Also, they're not likely to go after individual creators making tiny amounts of profit. But AO3 and OTW? Yeah, they're big. They're very much on the radar of a very large amount of commercial interests. So they have to walk a finer line and be more strict about things.
I'm cool myself with gift fanart; I may even commission a fanartist someday to draw some of my OCs (like Greer, she needs fanart). But not from someone who advertises in my comments on AO3.
Not related to that, but I sent out my yulecard card! I hope the recip enjoys; I ordered a special card and some stickers off Etsy from 2 different sellers, the stickers came last week but the card didn't come until Wednesday, and since Thursday was a holiday I put the card in the box for pickup today.