As we stated in another interview answer: Is this your first job? If so, describe how you have studied retail, and talked to friends and family about this job, and you understand customer service. With that, describe how you will hold your composure, you will remain friendly, and do whatever you can within company guidelines to help the customer have a positive experience.
If you have worked in retail before, then great! Easily give an example of when this has happened, and describe how you held your composure, and helped the customers needs.