Great feedback sets up a systematic link between the listener's past action, some consequence and your comments. In the case of positive feedback, the listener understands why and how his actions make an impact. In the case of negative feedback, he understands how he might be hurting the enterprise. By linking behavior and consequences, you're giving power to the person. The Ah-ha Experience.
Great feedback is educational.
The listener learns the consequences of his actions and alternative ways to do things in the future. As a result, the listener becomes more knowledgeable and insightful about daily behavior and activities. By learning an alternative repertoire, she becomes a more effective member of the team. Now you should be better able to respond to these customers. But good feedback is usually best delivered in a cordial, professional tone.