I'd be going with what some of the others have suggested.

Lodge an unpaid item dispute, don't agree to a cancellation.

It could very well be that the buyer is aware that a strike against her could affect her ability to buy from some sellers, so maybe she tries it on with a lot of people, to get them to cancel a sale rather than report her as a non payer.

Sellers can give her bad feedback with a positive dot till the cows come home and it won't make much difference except to alert other sellers after the event.

UPI-you'll get your fees back, she'll get a strike & it sounds as if she deserves one.