You know, Op.

This is isn't choosing beggars type shit.

This is straight up a malicious breach of contract.

If I were you, and I really didn't want to be fucked with again, I'd take this person to small-claims court, demand my $30 on principle, despite court fees costing much more than $30, then ask for punitive damages based on the severity of her behavior.

"But you already did it for free" is some bad faith, malicious, bullshit. And then asking you to do double the work in order to get paid for the first painting?

Oh hell no.

This would be an easy fucking 10 minute hearing. 2 minutes of you explaining the commission, showing the judge your messages (the contract), followed by 8 minutes of the judge tearing the woman a new one for being a dirtbag.

You're fucking grinding out art at $30 a pop.

If that is too much to pay, don't fucking agree to it.

I don't know what state you're in, but goddamn. I'd go for punitives, turn a $30 commission into a $300 commission via punitives for malicious breach, and then keeping it in your pocket to remind people who don't want to pay that you're not averse to making them pay.