If you are paying for something with your credit card outside the United States and a merchant offers to convert your purchase to dollars on the spot, think twice. This service, known as dynamic currency conversion, is usually a better deal for the merchant than the consumer.

Normally when you make a purchase overseas with a credit card, you pay in the foreign currency and Visa or MasterCard converts the purchase into dollars, adding on a 1 percent conversion fee. Your credit card issuer may add an additional 1 or 2 percent.

With dynamic currency conversion, the bill is presented to you in dollars and the merchant’s bank makes the currency conversion, using whatever rate it chooses, within reason. Usually, this rate is less favorable than what you would be charged by your card issuer, even with the additional fees.

Merchants are supposed to ask customers if they want their bill converted into their home currency. Unless you’re sure this rate will be better than what your credit card company will charge, just say no thanks.