I know someone who takes fandom commissions that aren't necessarily porn (porn is an upcharge), but does constrain the kinds of commissions she'll take.

Then, too, there's the fact that if you're posting on AO3 you can't say it's a commission or link back to your commissions, which is a completely valid choice on the archive's part, but also makes it difficult to raise awareness on the part of the commission-writer, you know?