I had an opposite (BAD) experience with Amex. I switched my internet provider, called the old one and cancelled the service. They didn't give me any cancellation number, but kept charging me monthly via Amex. They were going out of business (no wonder), so they stopped picking up the phone and had PO Box for the address, but kept charging!

When I asked Amex to cancel or prevent further charges, they refused to do anything without a cancellation number, which I didn't have and couldn't get.

So, your mileage may vary.