Worked at retail for years. Often the laugh is then coupled with outright ignorance like "Oh, I didn't know" as if that makes it okay. What I did was burn a few seconds at the start, on days I could feel it fighting me - "Hi, I'm \_\_\_\_\_\_\_\_ , I have a speech impediment so bear with me if it flares up." It's not an apology, its educating each customer on the situation in your introduction - They have no excuse for ignorance if they react because they have been told. You no longer need to worry about the moment they realise you have a stutter and often it helped me be more fluent in general as it shifts their focus - Anyone who then decides to be ignorant, or laughs I'd honestly ask "Oh, are you racist too? Sexist? You're laughing on that level, just so you know." Leave the shame with them, if they can't see it then clearly they're not worth your valuable time. Best of luck and don't let them get you down.