I worked in retail for a long time and i always just tell them on the first day about it and that it has nothing to do with stress or being uneasy but that its since birth and that the brain doesnt have some connections other people have. And normally they will be absolutely cool with it. Customers can be sometimes annoying but as long as the other employes know it will never be a problem. Thats my experience and i hope it will be as good for you too. Its not your fault you stutter. Its like having brown hair or blue eyes.