1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
<time you interacted> I <description>. I received what I felt was extremely poor service, and I felt I should write and inform you.
I would like to continue being a customer of <company name>, but I feel I can only do so if this situation is remedied. Please contact me and let me know what you intend to do about it, even if the solution is simply <solution>. Thank you for your attention to this matter.
encl: <List of enclosed items goes here>