Hi swaac-uwo-ca,
It's a pleasure to see you here. Welcome to Community! I can see the benefit of have the option to include AMEX as a payment method for your customers. However, this option currently isn't available for QuickBooks Online Payments Canada. You can review the acceptable payment methods here. If you would like to see the payment options expanded, please don't hesitate to submit a suggestion.
If you have any other questions, feel free to reach out. We will be happy to help!