Have any payments to American Express been made from other than your own personal bank accounts?
Are you the owner of, or employed by, a company engaged in a business that is legal in some jurisdictions, but not in others?
I wouldn't take this approach, until after consulting with an attorney and concluding that it is worth the time and expense. Once you mention this possibility to a large company it is likely the file will be forwarded to their legal department, and the character of the discussion becomes adversarial.
Point taken. My idea was that such a letter was a last-ditch resort before going to an attorney anyhow. Maybe turning it over to legal would get it closer scrutiny.