What does "cancelling the payment" mean? If you scheduled a payment in advance on the Amex website, you should be able to cancel it without repercussion if you do so before the deadline. After the deadline, there's no way to cancel it. The same would be true of a bank's bill pay service.

What I suspect is meant by "cancel" is something that caused the payment to be returned by the bank unpaid. It could be a bank bill pay check that was sent out and then the customer put in a "stop payment" request. Or, it could be an ACH transaction that they disputed and had sent back unpaid (generated either by bank BillPay, or from the Amex website). In any of those situations, it would be a matter of a payment actually being submitted to the customer's bank and then being returned unpaid. Even before the recent anti-money-laundering crackdowns that others have mentioned, creditors have for a long time been very uncomfortable with any situation that results in a payment being returned unpaid, much more so than late payments, and even in the case where the money wasn't even owed. For whatever reason, it just makes them nervous. I suspect that they have statistics that tell them that it's highly likely that there's some kind of financial problem when these things happen, even if they can't quite tell what it is. I'm not saying their reaction was correct or even reasonable, but it is how they seem to operate.