No, it's not a scam letter you received in the mail! Millions of Visa and Mastercard users are eligible for a cash refund of illegal transaction fees that they paid during overseas transaction. The original lawsuit was filed because these fees were largely seen as hidden from consumers and therefore illegally imposed. So, how much do you get? Consumers who spent a short time overseas are eligible to receive $25 with minimal paperwork. Those that spent longer than a week or spent more than $2,500 are eligible for a larger refund if they fill out the paperwork for it. You can find more information and a link to the forms here.

From USNews:
Tens of millions of credit card users received letters in the mail over the past couple of months informing them that they may be eligible for a refund of the currency conversion fees paid during overseas transactions. The settlement was the result of a lawsuit alleging that Visa, MasterCard, and Diners Club did not disclose the 1 to 3 percent fees they charged on foreign transactions.