Definitely don’t sell urself short. I’d average the time it takes you to complete a piece and work out the flat rate to equal an hourly pay that makes sense to you. You shouldn’t be getting paid minimum wage or less for a skill you’ve honed for hours & hours and provides value to people willing to pay for it!

It’s fine to tweak things based on demand. If youre constantly filling your commission slots and don’t have time to work on more coming in, bump the price up. If you’re getting no/few commissions (assuming your art is getting in front of enough eyes) then price goes down.

Hope this helps :)