This professional French-language study examines how customer service representatives handle complaint letters in organizational and commercial settings. Drawing on methodology and business French, it analyzes the practical difficulties, communication strategies, and adjustment-letter practices that arise when customers put dissatisfaction in writing. The work is oriented toward training and professional practice, clarifying the expectations placed on customer relations staff, the tone and structure of written responses, and the role of language in maintaining service quality. It treats complaint letters as moments where institutional voice, problem solving, and client trust intersect, making it useful for readers interested in customer relations, business communication, and applied French in workplace contexts.