By now, you know that plastic is the best way to pay foreign bills: credit cards for big charges, debit (ATM) cards for local cash. But a current scam targets each payment option.

New Credit Card Scam

"Dynamic conversion" is a popular scam that sellers use when you pay with a credit card. The seller offers the "convenience" of paying in U.S. dollars, which is really no convenience at all. If your credit card adds a foreign-charge fee, it will add it to any foreign transaction, in dollars as well as in local currency.

And the scam is that the seller can set the conversion at a really lousy exchange rate, as high as 9 percent above the bank rate, according to some reports.

Avoiding the scam: When someone asks if you want to be billed in dollars, no matter now much urging you get, say "no" to dynamic conversion.

New Debit Card Scam

Retail foreign-exchange outfits such as Travelex have signed deals with airports for exclusive ATM service. Although ATMs controlled by exchange outfits often claim "no fees," they catch you by converting at retail exchange rates, which can gouge you by more than 10 percent in some locations.

Unfortunately, these scamming ATMs are the only ones you see at many important international arrival airports. So even if you use a debit card with a low exchange surcharge and limited transaction fees, you can't avoid being victimized.

Avoiding the scam: Use only ATMs operatated by major banks, even if you have to wait until you're out of the airport.

More from SmarterTravel

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• How to Avoid Wi-Fi Scams While Traveling
• How to Protect Yourself from the Airport Bullet Scam

Consumer advocate Ed Perkins has been writing about travel for more than three decades. The founding editor of the Consumer Reports Travel Letter, he continues to inform travelers and fight consumer abuses every day at SmarterTravel.