Example of Modified-Block Format
The image below shows an example Complaint Letter.
Respect
Next comes the salutation. If you have a person’s name, great, use it. But if this is your first correspondence, you most likely will not have a name, so the most neutral greeting is, "Dear Sir or Madam." Right away, you are showing respect by using the honorific titles, sir and madam. Avoid the distant-sounding "To whom it may concern."
Next comes some critical information: After the abbreviation Re: or the word Subject, put your account number, order number, or tracking number of your purchase. Providing this info will save time by preventing the customer service rep from having to contact you and ask for it. This number is essential for the rep to provide you the service you desire.
The Body
The first paragraph should consist of a single sentence that states the facts about your purchase: the date, type of transaction, full identification of the product or service you paid for, and where the purchase was made. Here is a template of the sentence you should write:
On (date), I (bought, leased, rented, or had repaired) a (name of the product, with serial or model number or service performed) at (location and other important details of the transaction).
Next, in the second paragraph, explain the issue you are having—this is your actual complaint. You typically describe the fault, imperfection, or failure of whatever you acquired. The template for this paragraph is:
Unfortunately, your (product or service) blanked (failed, did not perform as advertised, is defective, was inadequate, is in error, etc.). I am disappointed because (describe the problem in detail: how the product failed to work properly, how the service was performed incorrectly, how the billing is incorrect, how something was not disclosed clearly or was misrepresented, etc.).
Your third paragraph states how you want the problem solved. In other words, this is what you want the company to do. The template for this paragraph is below:
To resolve the problem, I would like for you to (state the specific action you want—a replacement unit, your money back, a credit, repair, an exchange, etc.). Enclosed/attached are copies of my (include a copy of your receipt, guarantee, warranty, canceled check, contract, or any other proof of purchase and pertinent document).
In your final paragraph, you sign off with a calm but firm warning that you expect action on your complaint within a reasonable amount of time--or else. Do not threaten legal action at this time. Here is a template for the final paragraph:
I look forward to your reply and a resolution to my problem, and will wait until (provide a specific date 1-2 weeks in the future) before seeking help from the Bureau of Consumer Protection or the Better Business Bureau. Please contact me using the information above by (phone, email, or letter).
Next is the complementary closing and it should always be the single word: "Sincerely."
End with your name and the word Attachments (for digital correspondence) or Enclosures (for physical correspondence) if there are any attachments or enclosures.
Source: Bureau of Consumer Protection