Every payment in dollar has 1.5% IOF (Tax over financial transactions) + The rate used is the conversion rate valid when you pay your credit card.
Paypal + Visa (My Setup): VISA is debited on BRL (Paypal converts to the national currency) so no taxes and the conversion rate is the rate of the day of the purchase. You get to pick better rate's, and the conversion rate is better than VISA's.
Note for AMEX users. Everything you buy which requires a currency exchange is made on a somehow not very lame daily quotation (like EUR to USD). Everything you buy with AMEX however gets charged addittionally by 3%. You have to login on amex page and click a bit to find the fee robbed by amex. It's the most expensive card to have!
Summary:
mastercard is the best online shopping companion usually.
Not sure what it's like outside the US but at least in my case, it's the credit card issuer (ie. bank, credit union, etc) and not the credit card company (Visa, Mastercard, Discover, etc) that ultimately determines what fees apply to foreign currency transactions. Visa and Mastercard typically charge 1%, flat and use a less favorable rate of exchange compared to mid-market rates. So unless your bank decides to eat that 1% which some do for particularly profitable customers, you're pretty much stuck paying 1% but on top of that, many banks tag on an additonal fee. For retail banking customers, that's often up to an additonal 2%. Some banks charge a flat fee in addition to the 1% "transaction fee". If you don't live in the US or in a country that uses the US dollar as its main currency but do a lot of online shopping, it's probably a good idea to shop around for a credit card because those fees do add up rather quickly.
AFAIK, you can decide at payment verification whether PayPal debits the credit card in foreign or local currency. I usually go with foreign currency, as Paypal's exchange rates are always significantly (4-5¢/€) worse than the ones the credit card company will use.
i have had no such fees EVER charged on my Discover (which is not like Visa or Mastercard)
HOWEVER, they're current rates are just way too high (apparently they think my credit has gotten worse. yet the scoring websites say exactly the opposite. bout to drop them and go with a more local visa)
also, i use paypal for ANY transaction overseas. its just better to be protected by them!