Answer:
In a complaint letter, the customer must collect sales receipts, warranty receipts, contracts, messages exchanged with the company or the seller and any type of document related to the purchase.
Step-by-step explanation:
When issuing a complaint letter, the customer must have any and all types of documents or media that justify the complaint and prove that previous attempts to solve the problem have not been resolved. In this case, the customer must include in the letter any type of documentation related to the purchase, invoices, warranty notes, messages exchanged with the store and the seller and any type of service order.