Even if they're yelling - Get past the volume and really listen. The sooner you're able to find out what the problem is, the sooner you'll be able to take care of it. More often than not, a customer is really only asking for more information. At the same time, don't think they don't know quite a bit. Be mindful of what you say and how you say it.
Most customers are people first, who are aware of the realities and limitations of doing business.
And most of them are understanding when things don't always go their way. You really can't please all of the people all of the time. No matter how hard you try. And know that there will be times when you will have to risk the loss of a customer. All you can do is trust in your decision, and hope they will not be able to severely affect your sales because they are being unreasonable or unruly.