OP seemed kind of snotty from the get go.
Polite and professional would have been “For that price, I can offer a 13x13” instead of what she said.
Polite and professional isn’t “not my problem,” it’s something more along the lines of “for that budget, I can offer two 8x8 with different poses” or simply, “I’m sorry, but I’ve priced my commissions as appropriately as possible considering the quality of both the materials and the labor.”
Choosing beggars suck, but I thought OP was kind of snarky from the start. I could see getting bitchy if she got nowhere after being polite, but she fired it up right out of the gate.
And how did she know the person texting her was a “ma’am?” I’m a little skeptical.