If you are intent on continuing your buisness there, then the next time you hear it, you just kind of ask "I'm sorry, what was that last part you said?" Then depending on how the answer goes, you could say "it's mam", or if if they say nothing, you could laugh and say "oh I'm sorry, I thought you had said sir"....
If that doesn't get their attention, then I would go to a manager, or if on the reciept you have the option of emailing how the empoyee did, then I would utilize that.