Australian dollars (AUD) are the default currency. US dollars (USD) can be selected on the order page.
Choose the currency before adding products to the cart, then review the currency shown beside every price and in the order total. The invoice records the currency used when the order was placed.
Your bank or card provider may perform its own conversion if your payment account uses a different currency. Any conversion fee comes from that provider, not from the currency selector in the client area.