There are a lot of ummm, uncouth people in this world and as a businessperson, you will run into more than your fair share of them. Developing a thick skin would be a good idea because otherwise it will only make you look bad. People dont remember the rude, uncouth customers, but they will remember the counter-person or manager who shows his temper in return to these type of customers. Every once in a while you will be justified in, shall we say, returning fire but the better course of action would be to calmly explain the situation to everyone's satisfaction by revealing the facts.