You are right in that she does not have to grant my request. And, likewise, I don't have to like nor respect her decision. Thus, I have the right to post a complaint and expose what she did to the public as a warning for future buyers.
There has been a lot of talk about negative versus neutral. I think the size of the over-charge plus her less-than-polite attitude more than justifies Negative feedback. Neutral feedback is mostly ignored. However negative feedback catches one's attention. She will continue to receive positive feedback in spite of my warnings -- so long as things go smoothly. But just wait for one bad sale with another customer and see which personality of hers surfaces. She is not customer-oriented and will not do what it takes to make things right