You can only make a payment via the website/app once every 72 hours. A lot of the time though it still won't let you. You can call Amex up and they can process the payment for you but they are still bound by the 72 hour limit. To get around this they can process the payment onto a supp card for you.
Benefit of doing it this way over BPAY is the funds are available instantly.
Benefit of doing it this way over BPAY is the funds are available instantly.