They charge a fee per transaction. I've just run one and there's a 2.1% payment charge (which, depending on your circumstances, may or may not be GST deductible).
Amex FX have me a USD exchange rate of 0.668 which compares quite nicely to the 0.647 Westpac offers today.
Our circumstances are a bit weird as we need to make a payment in USD to an Australian bank account, no SWIFT, just BSB and Account Number. Banks struggled with this and I don't think RewardPay would have allowed it either. So it was kind of a last resort that led to Amex FX, but it works.