Umm what I think ya might want to talk to HR or just don't talk to anyone

customer service, where no matter what you do... you're wrong

The current world where being polite is odd... Be brave, people.

Growl at her next time and say you hope she never comes back. Or maybe just deadpan face. But seriously I'm guessing your place, like pretty much all workplaces, has tons of cameras. Request that the footage be pulled if she's trying to make a legal case against you.

I'm sorry that all the /r/niceguys in the world messed up customer service.

Maybe switch to "have a good day" or some such variation instead.

Regular customer means she would see you soon, where do you work? Perhaps just say "have a nice day"...

Next time look at them dead pan and tell them you love what they’ve done with their living room