As customers, we aren't the most memorable things to the employee. When I was a cashier I pretty much immedieatly forgot the customer once they left, unless they were rude or creepy (or did something really nice, on the rare ocassion). Having trouble getting their words out was the least to ping my radar. It's mortifying to mess up while ordering, but stay polite and don't beat yourself up over it and you'll get through it. It's not as big a deal to observers as it feels like it is to us. Hope that helps, it usually helps me prep myself before I need to talk to an employee.