When confronted about not paying originally, I would have stumbled, blushed, became very embarrassed and get slightly shaking hands. It's my reaction every time to finding out I've made a mistake- whether the mistake was actually mine or not.
I then would have almost thrown my card at her to pay.
I would have walked out ashamed and need to sit down for 10-15 minutes to calm down and figure out what to do next.
It would hit me at some point to check my own bank statement, but I can assure you that little strike of inspiration would come afterwards.
I'm damn well 31 and thought I would have grown out of reacting like this, but apparently not.
So, I say good on you for even refusing to pay after her demands. I can't actually say for sure what I would have done afterwards because I'm not so gutsy- most likely a.
Either way- dont shop there again!