State very specifically what the problem is. Asking them to replace the seat would be reasonable. Enclose any receipts, product codes or numbers, proof of purchases, or any other identifying materials. Most companies can sniff those letters out immediately. Only write with a legitimate complaint or compliment.
Students use their home address as the return address and in the heading.
Therefore, the return letters come to their homes. I usually provide stamps for some students for whom I know this will be a problem. For the most part, the students are very good about bringing in their own stamps. Then, I mail all the letters to assure they do make into the mail. It's a lot of fun for the students to get replies back from the companies.