Theoretically there is a third option, stay in the store near the cash register and call the police to come deal with it on the spot before the purchase. The problem is that they probably won't bother coming, and if they do, they won't come quickly enough to make it worth waiting for them given the amount of money at stake.

Edit: Yeah, I did say before the purchase, but I should have said after the purchase when they pay the legally correct price but the store accuses them of shoplifting and tries to detain them. And I know it's often infeasibly hard to pay the legally correct price from a logistical perspective without the cashier's cooperator, especially if you want to pay with a card. It is clearly possible to put at least the right amount of cash on the counter, ask for the change, and attempt to leave if they refuse, but that doesn't guarantee ever getting the change. Anyway, I did list this option as (purely) theoretical and not as actually practical.