Do not waste your time on the private entity known as the Better Business Bureau [BBB]. It takes a hell of a lot to get a bad report from them, and good reports come with membership.
Instead of complaining to a non-starter, complain to the U.S. Postal Service, for a demand for money not owed. Those who say you do not own for unsolicited things received by mail are correct.
You can also complain to the Federal Trade Commission [FTC]. They do act on complaints of acts done via the U.S. Mails and wires.