I have worked retail and have been assaulted by customers. If you want your job you have to take it. I did not quit on he spot but that moment lead me to go back to school and seek new employment. Yes, you could have handled that better. but i understand it. The best response is to apologize, and then request that a manager or other associate deal with the customer Basically say that you will not serve her after that, but she is more than able to seek the assistance of another. If your boss does not like that - you trying to remove your self from a hostile situation while getting the customers money-there is a problem with management. I've know some complete jackwads who won't fault you for trying to gracefully exit..
"But as always, technology refused to be dignity's bitch."--Vance DeGeneres
