This letter is a follow-up to the first product complaint letter, to which you received no reply. Be firmer but do not threaten...yet.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
<time you wrote the letter> I wrote you a letter expressing my dissatisfaction over your <product name>, and requesting a suggestion of action from you, either a refund, a replacement or a referral. Since that time I have received no reply of any kind, so I am contacting you again.
I wish to remain a customer of yours, but I am currently very unhappy with the purchase of what I feel was a product of poor quality. Please respond immediately so we can resolve this matter to both <company name>'s satisfaction and mine.
I look forward to hearing from you.
encl: <List of enclosed items goes here>