Let's put a modern spin on the event. If I go to a professional photographer who specializes in portraits, I demand that he does a job that makes my person stand out, showing his skill (otherwise I would take a selfie and that's it).
But if after two hours of work all I can come up with are banal photos full of Instagram-style filters, I obviously don't pay him, I send him to hell and I also put a negative review.
So, that's what happened to the protagonist; she was expecting a portrait from an artist she knew to be great and she ended up with an IG selfie.
Also
1. it was the artist himself who told her "you decide how much to pay me after you see the work". And since the MC didn't like the work and didn't take it away, she has every right not to pay for it.
2. just as the artist lost two hours of his life drawing, the MC lost two hours of her life modeling him still. And who's going to give her back those two lost hours for something she doesn't like?
p. s. The world of professionalism – especially the arts – is horrible, but that’s how it works. If the clients doesn’t like your job, no one buys it. Don’t you have any food? It’s your business, you could have worked better.