The Fit Mastercard exchange rate is Mastercard's exchange rate on the date you make a purchase, plus any foreign transaction fees. While Mastercard exchange rates change daily, the Fit Mastercard foreign fee is 0% the 1st yr, 3% after.
It's best to use a credit card with no foreign transaction fee when making purchases from international merchants. As long as you have such a card and avoid dynamic currency conversion, the exchange rates you'll pay on international spending with a credit card will be much lower than those you'd get...