A couple weeks ago I purchased a flight ticket from Amaszonas Airlines, a Bolivian company, using my Amazon.ca card. Today I received the statement and did some calculation and research. The transaction is 2156 BOB (Boliviano, the currency of Bolivia), I was charged CAD$410.13 at the rate of 0.19022727.
From Google I found the middle rate on the same day was 0.1890. So the difference is only 0.63%. Adding the fact that this card does not charge foreign transaction fee, that turned out to be a really good rate!
Conclusion: If you need to make some purchases abroad, the Amazon.ca card may be a good choice to save on exchanging cost.