Your SimpleTix site can work with all currencies, which are set as a culture setting; all dates and currencies are formatted based on this culture.

In this example below the site is: en-AU for English - Australia.
Notice the money amounts have the $ symbol, similar to the USA dollars.

You cannot change this formatting. For example you cannot change it to: $20.00 AUD
However, it is possible to make a note on the footer of the shopping cart page which notifies the user that the $ symbol represents AUD not USD for this shopping basket. Here's an example:


There is a Site Setting called "Order Total Footer" by default it's blank.
However you can change this in Store Information > Store Settings > Order Checkout.

Sample Case:
One Danish client's site uses "Kr". However  for legal matters it is necessary to specify “DKK” because other neighboring countries (Sweden and Norway) also use “kr.”. So in this case you can change set "Order Total Footer" to say "All prices listed in Danish DKK"

Here's how the client designed this following the instructions above: