How to prevent currency change during one page checkout?

In the one page checkout a checkout restriction applies that changes the currency to match the shipping destination. Is there a way to stop this and display the selected currency?

Many of our customers are sending gifts from one country to another so they may, for example, be shopping in Australia but shipping to USA. Currency display is set to $AU by IP address or manual selection. But, when they checkout the checkout restriction changes to $US.