Listen to the customer, apologize and fallow company procedure. If the situation gets to a point where I'm unsure, I would find a peer who is available and ask them for assistance. I would listen to my peer assist the situation so I would be able to handle a similar complaint in the future without assistance.
As a team member at Tim Hortons I did this by being friendly to the customer ensuring that her order was complered accurately and in a timely manner. If a customer is a regular I also already know their order and thier name