I was asked this question during an interview for a Mail Processing Clerk position. My answer is to LISTEN, and try see it from their point of view. Try to neutralize the situation and come up with a solution. Or ask the customer what would help them feel more satisfied. Turn it around to become a pleasant end to a difficult situation for the customer. If there is no calming to be had, ask if the customer would like to speak with a supervisor if one is available.