Unless you are flying from one of UK or Canada airports, all prices are displayed and quoted in U.S. dollars (USD) and all charges made are in U.S. dollars (USD). If you are using a non-U.S. credit card, you should check with your bank for details on currency conversion charges. Your non U.S. credit card may charge you a fee for processing purchases made in U.S. dollars.
If you have searched a flight departing from Canada or the UK, you will see the prices in Canadian Dollars and Sterling Pound respectively and the charges would be made in respective currency. If you use a non-Canada card for Ex-Canada booking or non-UK card for EX-UK booking you should check with your bank for details on currency conversion charges.