Proofs can be attached with the complaint letter
For strengthening the value of complaint letter proofs can be attached with the complaint letter. These proofs can be the copies of purchasing receipts, product warranty if any or any other similar kind of document which can serve as a proof that letter writer has made the purchase.
Proofs are not mandatory to be attached with the complaint letter but they can be good and supportive in proving the point. Also things get easy at times for the reader when he/she has proof at hand.