Sorry if this particular language was already covered. As someone who has lived abroad and visited Austrailia a few times, they are telling you if you bought $1850 AUD using USD, for which you will pay in USD base on the exchange rate, senidng them the $1850 AUD is all that's needed.
We used to have to "buy" UAD at the exchange if we wanted cash and had to pay crazy fees. As others have already said, using a CC will fix it all for you automagically. You'll see $1,186.96 USD go away and they'll see $1850 AUD show up.