Tell the people at work that you stutter. Part of your anxiety will be your secret being revealed and uncertainty about how that will go. This should stop the laughing and jokes as your colleagues will understand that you have a genuine condition. Generally talk to your manager about your worries. This is why they are there. You got your job because you are a kick ass bank teller. What specifically is worrying you about these drive through customers? What could they ask that will cause you problems? How can you prepare for that? Are you actually worrying about nothing? Confidence goes a long way and a simple thought of “I will provide the best customer service this person has ever seen” before they approach likely will go a long way.