Let the Better Business Bureau know about a grievance against Amazon that has gone unanswered by the company or that it has responded to unsatisfactorily. Complaints can be made to the agency either through its website or by calling its local office. That's what the BBB is designed for: to pass on complaints to the companies in question and try to resolve the issues, without them requiring further mediation.
Write a letter directly to the corporate office at Amazon. Find out the names and addresses of members of senior management who can provide resolution to the issue, or who could at least provide guidance about where to turn.
Contact the Federal Trade Commission. If an issue is deemed serious enough and it's felt that other people have also experienced the alleged injustice, or may be impacted by it in the future, then consult the FTC. The FTC filed a complaint against Amazon for deceptive billing in the past, so the federal agency may be helpful.
Contact a public interest group and add to the chorus of others complaining about the same or a similar grievance. The Institute of Public Representation filed a complaint against Amazon in 2013 due to grievances from the deaf and hard of hearing consumer groups.