Make default currency in checkout AUD only?

I'm an Australian retailer so I would like all my orders to be checked out in AUD only. As I have some overseas customers, I've put in a currency converter so that the overseas customers can have a guide to what the equivalent price would be in their local currency (USD, GBP, EUR etc.).

However, some customers have been allowed to check out in USD and these invoices are shown in USD.

Is there a way for checkout to be in AUD only and still have a currency converter (as a guide for overseas customers)?

Go to:

Localization > Currencies

The click on the AUD it is defaulted to the top of the list.

Then click on Edit. At the bottom of editable area click on the "Set as default (requires a manual update of currency values)" then Click update.

After this, click on the "update currencies" button.

You will the see that the AUD is updated to 1.00000000 value and it will then say Australian Dollar (dollar).