I was a supervisor at a grocery store one time and i have a moderate stutter that can get really bad depending on how I’m feeling. Being a cashier helped me a lot with my stutter. To be honest, most people don’t care if you stutter. Just relax, have fun, and be yourself. The more you bring yourself out, the less you’ll stutter and you’ll be more comfortable in these type of situations. If you feel yourself getting nervous or your stuttering getting bad, you can nod to the customer, give them friendly gestures, or just tell them you have a stutter. Also just tell your boss. Its not your fault you have a stutter, I bet after you tell her she’ll be more understanding and accommodate you if you really help.