I've been working in retail for 11 years and I deal with this same problem. As an AM of a department I'm expected to do phone calls. Which often leads to issues. One day, a few years back, while having a particularly stressful day (which affects my fluency) I received a call from a woman asking about the availability and price on an item. I stuttered during my reply to which she said "spit it out!" and then laughed. It still amazes me that people can be so ignorant about stuttering and people with the condition.