I understand your anger completely, no one should mock someone's way of talking. I started working in retail on tills just over a month ago to become a more confident and coherent speaker. Because I usually say the same things every time e.g. . 'Hello how are you, would you like a bag' etc I don't stammer. But when they ask me to repeat it I get a quick pang of anxiety, avoid eye contact and repeat my words again but this time I'm stammering. It can be frustrating but I've had no negative responses at all so far. Some customers suck, but then you get those that are patient, kind, and friendly to you that remind you there's good people in the world. Those customers that laugh at you will forget you even have a stutter when they leave the store. You're doing great and don't let those minor instances shut your voice down.