3
Be authoritative. Being authoritative in your letter creates the right tone and lets the company know that your complaint is to be taken seriously. This is especially true of more serious complaints, which may have considerable financial implications.[1]
Being authoritative encompasses a range of things, such as the quality of the language used, your knowledge of your rights and the company's responsibilities, as well as the professional presentation of the letter.[1]
All of these things give you credibility, which should positively effect the response to your letter.