Store currency vs Paypal express currency

Hi

I have a South African store set up and I display all amounts in ZAR. Obviously Paypal does not support ZAR and I am using USD for Paypal Express. The issue I have is that the order total in ZAR on the website gets passed to Paypal in USD, which means that if the cart total is R100 (ZAR) you then get charged $100 by Paypal. I've tried changing the various totals in paypal-express.php, but because it uses the global cart variable I get errors that the totals do not match when trying to direct to Paypal Express. All I want to do is convert the totals before sending them to Paypal. This should be so simple, but I just can't get it to work. Any ideas??