Yes, you could. Make sure the address of the Amex card is the same as your self-serve account if you are recently become a Koodo customer.

It depends, but it's highly unlikely. 

A US credit card can be used but koodo does have strict restrictions and if you are a newer customer and if the address of the US credit card does not match the address on your account the card it is more than likely that it will be rejected. 

@Bbbailey28 Given the tenure of your account using a US credit card to pay your bill online should not be an issue.