if she offered $100, and you have not already said $85, then let her pay you $100. tell her you made a point to add the right amount of quilting to make it worth her offer. that is the absolute truth so you need not feel as though you're doing anything unethical.
another truth is that she sees value in your work that you have not yet allowed yourself to see.
if you have already quoted $85, then of course you must stick to it. a deal is a deal.