Respond to a customer complaint letter with an empathetic tone that is concise and apologetic. Avoid making excuses or being defensive, and let the customer know his opinion is important.
Begin the response by thanking the customer for the complaint. Let the customer know that his complaint is an important issue that needs resolving so that the company can learn from the situation and future customers can benefit from the resolution. Mention that the company appreciates the chance to satisfy their customers. Keeping the tone of the letter personable generally means a great deal to the customer.
From there, address the specific complaint. Restate the issue at hand, and discuss what should have happened and the actions the company plans to take to ensure the problem does not repeat itself in the future. Lastly, provide the customer with something for taking the time to raise the negative issue, such as a refund or another type of compensation for the customer's effort.
It's helpful if the company responds to the customer in a timely fashion. The faster the customer responds, the more the customer might appreciate the gesture. If you take too much time to respond to a complaint after it is lodged, this can lead to even more negative feelings.