Simply replace the unfilled areas such as "" and "[date]" with your information.
Thank you for your letter of [date] in which you informed us that [product] you purchased was [nature of problem].
We are sorry that you experienced this problem and are enclosing [nature of compensation]. While we make every attempt to ensure against defects in our products, said defects occasionally arise.
We are very appreciative that you have taken the time to bring this to our attention and would like to thank you for your continued business.