That would be nutty though, verging on confusopoly for the customer and inconvenient for the business. (It would potentially necessitate system changes to accommodate that - and that would represent a cost, ultimately borne by the customer, and a delay in implementation, a delay that is not lawfully available to the business.)
I think businesses may start to demand that you nominate a payment method and stick to it. (If you want to vary the payment method then you would normally have to do that before the bill is issued, or if after the bill is already issued then probably via customer service to get the bill reissued.)
A slightly less tedious way of handling it might be … the bill is for $105 and you get a discount for all other payment methods except Amex and you pay $105 regardless of payment method and the discount, if any, shows up as a credit on your next bill. So, for example, if you actually paid by BPay, you still pay $105 but your next bill will be $105-$3.
It’s still confusing. It could generate some goodwill though in perpetually giving your customers small credits (which is actually the customer’s own money). If anyone complains, the telco can just blame it on the stupid government (with partial truth).
Amex is not a great example though since, as I understand it, they are still allowed to surcharge Amex.
