Maestro debit card is a pre-paid debit option. The option is in direct competition with paper travelers checks. Early roll out did not indicate a fee on top of the Mastercand Exchange rate. There are now fees on International purchases.
Different Mastercard member banks have very different
fees for international transactions. Before there were no foreign transaction fees but because of banks' appetite to generate more fees, some banks have started charging a higher than market exchange rate.
Merchants that sell merchandise will be paid in their home currency and home currency ONLY. As opposed to Visa which stipulates an option to convert currency and then charge, Mastercard constrains merchants to be paid in their home currency.