RE Amex at Post Office
You can pay your Card Account in person with cash or a cheque at Australia Visit to set up Post Offices.
Cash payments are limited to no greater than your current account statement.
You must present your card and current account statement when paying your Card Account.
A $3 processing fee will apply to payments made in person.
the best way would be to go in with a bill and try with self service and see shat happens
it also says fee will to payments, I guess that could be interpreted as $3 per transaction , so whether using mutliple cards would mean multiple charges, I have no idea,
it seems no one has tried that yet, I will give it a shot next month