I would suggest tht you address any letter you send to the company president / chairman / chief executive. Customer service managers are rather averse to being chewed out by senior executives because they are giving bad service. Had problems with my telephone company - endless promises from customer service that the problem would be sorted immediately - a letter to the chairman elicited a call from their customer service manager two days later, and resolution of my problem by the end of the week.

Cheers
Marco