That isn’t necessarily true. The risk to the institutions involved from adverse currency movements is proportional to the amount, the cost of insuring (hedging) that risk is proportional to the amount, as is the risk of credit card fraud (to some extent). For the actual mechanics of doing the transaction, I dare say that you are right - unless there are rules in place that all amounts over a threshold amount require additional checks.
I think that “everyone” agrees though that when you are told an amount that absolutely is in Australian dollars but the transaction is then done overseas / in a foreign currency, which causes you to incur a foreign transaction fee … that is a scam e.g. spotify. That does NOT apply to the OP - because if the web site had not been dodgy then it would have told the OP that the amount is in US dollars and the OP could then reasonably expect to pay an overseas transaction fee.
There is every likelihood though that if the dodgy web site gets changed to quote an amount in Australian dollars then it too will fall foul of the previous paragraph. In other words, it could move from one dodginess to another dodginess.
I think both of these areas need looking at by the Australian authorities.
