Robert Martin allowed a business associate, E. L. McBride, to use his American Express Card in a joint business venture in which they were involved. He orally authorized McBride to use the card to charge anything up to $500. Martin received a statement from American Express 3 months later; the amount due on his account was $5,300. Martin refused to pay, claiming that he had not signed the invoices, and therefore was liable, under the TILA, only up to $50 for "unauthorized use" of the card. American Express claimed that McBride was an "authorized" user and sued for the full balance of the account. Who won this case, and why?