NEW YORK ( TheStreet) - American Express ( AXP) is a leading financial services company offering credit and charge card products along with travel-related services to consumers and businesses across the globe. The company is the third largest issuer of credit cards in the U.S., after Visa ( V) and MasterCard ( MA).Our price estimate for American Express' stock is $47.16, about 5% ahead of market price. A key revenue stream for credit card companies like American Express stems from the commission charged to merchants in a card transaction by the card-issuing bank, otherwise known as the interchange fee. The Durbin Amendment, enacted in July 2010, granted the Fed the power to control the levels of this fee. In December, the Fed enforced one such limit -- 12 cents per debit card transaction. This limit could represent a loss of $13 billion in revenues for banks issuing debit cards, as per a report by CardHub.com. To compensate for the lost revenue, banks are expected to increase monthly fees on the corresponding checking accounts by $4 to $5 in addition to reducing reward programs on debit cards, which are typically funded by interchange fees. While this does adversely affect the two largest card payments-processing networks Visa and MasterCard, we believe it presents an upside opportunity for American Express. So, what differentiates American Express from the rest of the herd? American Express does not issue debit card products and limits its offering to credit and charge cards issued by itself and other third-party financial institutions. We previously discussed a report indicating that the number of credit cards in use in the U.S. declined by 8 million over the past year, from 70 million to 62 million. As most banks already wrote off bad debt during 2008 and 2009, the decline in credit cards in use is unlikely to have come from card cancellations. Rather, the decline is likely a voluntary decision by consumers to take on less debt and could signal an increasing consumer preference for debit cards. Debit cards have gained market share in aggregate transaction volume at the expense of credit cards since they offer similar payment convenience and rewards benefits without the interest due on outstanding balances that is typical of credit cards. Introduction of the Fed's interchange fee limit on debit card transactions, and the corresponding loss in revenues for card-issuing banks, will likely reduce available rewards benefits to debit card users. The result will be a recovery in the relative attractiveness of credit cards, triggering upside for American Express.