For the folks thinking about the "they just don't want the hassle of changing the order"/cash versus credit card thing, I could have also shared a very similar example at Chipotle. Same girl, Stephanie, wanted a burrito bowl, but wanted the onions and peppers added on top. Back then (not sure they still do this) they always said, "that's a fajita burrito without the tortilla" and instead of the $5 for the "bowl" she'd have to pay $7.

So I reminded her about the sub place. The next time she went to chipotle she ordered the bowl, and then waited as they were making the thing and while it was in the middle of the line, this time she would ask as an afterthought can they add peppers and onions, and they never again raised the $7 versus $5 argument.

And if you know Chipotle, you pay AFTER your order is complete, so they could have easily charged her extra without any hassle. But I believe just like the ice cream example, since they already started making a bowl, they had it in there head that they were committed to making the $5 bowl.