There's only one response to an attitude like this, which is, "May I speak to your manger please?" Or you can do a citizen's firing.
Happily I have yet to have an experience like this, but I have a terrible poker face and I'm sure I would look at Snippy Young Salesgirl with a look of incredulous disdain if she tried to pull something like that. She must have a terrible life if she's alienating *a customer.*
