Just to restate, when using your debit card if given the option to pay in US Dollars - don't. Pay in the local currency.
The same is true for credit cards - always pay in local currency and let you bank do the exchange.
If you pay in US dollars, the seller sets the exchange rate - it won't be in your favor!
EXCEPTION: Some credit cards have high fees for currently exchange, but most good credit cards have little or no exchange fees. Check the fine print for your specific card.
-Paul