Thank you for the clarification. I think its likely that American Express reacted to a large payment from an unfamiliar account, rather than to the method of payment per se. I wouldn't bother with wire transfer if there is a circumstance where you cannot pay through the americanexpress.com method.
Yea my suprise really was just that rather then ask me, a good standing customer since 08, they would just cancel everything. I mean if I was them and I was worried about the validity of a payment, I'd ask the customer. Instead they waited till they had my money and then gave me the boot. In the future I have no plans to wire, only did in this instance as they would not permit a second payment from the new account until the next bulling cycle.