This letter is a follow-up to the first and second product complaint letters, and expresses your anger at having been ignored. This is the time to threaten further action.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
<time you wrote the letter> I wrote you a letter requesting a reply to my first letter of <date of letter>. In both letters, I believe I made it very clear that I am very unhappy with the service I received <description>. To date, I have received no reply of any kind from <company name>.
Your complete indifference to my dissatisfaction makes it easy for me to simply never do business with you again. However, that does not free <company name> of its obligation to improve its service. I would like to receive <what you expect from the company> <time expected>. If I do not receive such information, I will be taking my case and my story to <advocate or bureau>.
encl: <List of enclosed items goes here>