It's time to change persona! You are now a representative for the company your complaint letter was addressed to!

Reply to the letter in the (formal, businesslike) tone of the company. You have three options: 

• Redress the complaint ("oh we're so sorry, let us do xyz to fix it!")
• Reject the complaint/responsibility ("not my problem, lol")
• Pass the complaint and responsibility up the chain ("Not my division, have forwarded your complaint to the right people, they'll get with you asap")

Your response should be properly formatted as a letter, should address all the points from your complaint, and should be written in the appropriate tone.