A complaint letter is a letter that serves as a written complaint. This can be used to issue a complaint regarding poor services, poor management, poor product, or anything. Usually written in formal language and is used to inform about the issue you had.
Product complaint letter sample:
To,
___________
___________
___________Date: __/__/____ (Date)
From,
___________
___________
___________Subject: Complaint regarding ________ (product details)
Respected Sir/Madam,
I am writing this letter in reference to the order I received on __/__/____ (Date). I placed an order for ___________ (Mention ordered item) on __/__/____ (Date) but the delivered item was found to be ____________ (Damaged/ Broken/ Not working). The payment of the above mentioned order was done ________ (Online/ Cash on delivery).
It is to inform that the experience was not upto the mark and the product receeived was not in acceptable condition. It is requested to kindly look into the matter and resolve the issue at the earliest.
Regards,
___________ (Your Name),
___________ (Contact Number)
lettersdadmin 
Rahul Sharma