They really don't (unless you compare them to PayPal, I guess). They take 2-3% of commission to convert your funds, which would mean that they charge you $200-$300 for currency exchange on a $10,000 payment.

If you want to convert your funds at a "Big" Canadian bank, please do so at Tangerine, which has the best rate I found (they take 1.45%) - otherwise, there are plenty of other suggestions in this thread that could help. The best rate will most certainly be with Interactive Brokers (max(0.01%, $2.50)), but the account set up is a bit tedious and they have a few restrictions.