Customers sometimes don't understand what they're buying. They may misunderstand product capabilities, delivery dates, return policies, etc. Acknowledge receipt of the complaint and then clarify the misunderstanding. State the possible remedies, if any. The letter should be cordial but firm.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I was sorry to hear <nature of problem>. I'd like to clarify. <Clarification of your position.>
<Optional offer of alternative.>
I apologize for any inconvenience this misunderstanding may have caused. Thank you for understanding. We look forward to serving you in the future.
encl: <List of enclosed items goes here>