I'm sorry you have to go through this. When you signed up for this job, it was not in the job description to be mocked and it takes a lot of courage and confidence for you to continue working there. People suck sometimes; have they not been taught basic human decency? You really should stand up for yourself and ask them what's so funny? You're serving them food, this is a transaction, it's not meant to be a spectacle. If they wanna be served by someone else, they can get in line at a different cash register. Don't let miserable people affect your life too much. Whatever you do, breathe and repeat what you wanna say in your head. Have a line that you repeat in your head and then say it out loud and you probably won't stutter. The more you repeat it in your head or out loud (at home) the less likely that you will stutter on it again. I hope things get better for you!