See "Response to Final Notice Before Legal Action" in the Legal Matters Bundle for a slightly different way to handle this situation.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
I was sorry to hear <nature of problem> <Elaboration of known facts.>
<Description of your position.>
<Optional meeting proposal.>
<Your credentials and conclusion.>
Thank you for you cooperation in this matter.
encl: <List of enclosed items goes here>