You're trying as hard as you can to say the words but you're trying equally as hard to make sure you don't show any stuttering. The result of these two counter forces pushing on each other is you get stuck. The options as I see it are not to stutter (which usually involves not speaking when you can't switch words) or say what you need and let people see you stutter. Now how to actually be able to open up and allow yourself to show stuttering is definitely a journey, but I hope you know you can show stuttering in front of customers (not blocking but letting go of trying to control) and it's totally fine. They won't care that you stutter, just that you give correct change.