The best businesses realise that at some point in time, they will receive complaint letters, and customer service etiquette uses the response letter to improve the company while keeping customers content. Regardless of the complaint letter's nature or tone, the patron deserves a courteous and prompt response, if he signed the complaint and provided a return address. The correspondence becomes a permanent record for audits and court cases.
Summarise the complaint to allow the reader to see that you understand the customer's complaint. Repeating the complaint does not mean that you agree, but it shows that you listened and understood. State that you empathise with the customer's frustration.
Explain what went wrong, or why it happened. If you need to investigate, advise the customer immediately and assure him that you will provide updates during a lengthy investigation. Do not forget to send those updates.
Tell the customer how you have resolved the problem or plan to stymie it in the future. Answer each complaint in the customer's letter. Do not skip or ignore any issue.
Offer the customer something, such as an apology, gift token, discount on the next order, a refund or priority service with the next order. Customers need to know that you appreciate their patronage and time taken to write.
Close the letter by telling the customer what to do next, such as writing again if dissatisfied in the future. The customer needs assurance and a clear idea about future action. End with action rather than vague generalities.
Avoid using "I" in the response letter, because the letter's focus should stay on the customer or the business itself, not on the writer. No matter how negative or angry the complaint, do not respond in kind and never, ever name-call. The same writing rules apply for e-mailed correspondence. Do not discard the complaint letter after answering it. Keep a copy of your reply.