Once you become a cardmember, American Express also has a full array of online customer services.
Those who have store branded cards may send Chase payments to Cardmember Services, P.O. Box 15148, Wilmington, DE, 19886-5148.
Then mail it, along with the check, and a copy of your driver's license, passport, Mexican Consular ID or non-driver ID, plus a document proving your address to Citibank New Cardmember Services, P.O. Box 6264, Sioux Falls, SD 57117.