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 US, after Visa ( V) and MasterCard ( MA).Our price estimate for American Express' stock is $47.16, about 10% ahead of market price. American Express predominantly makes money by offering credit and charge cards to consumers, earning a percentage commission from the merchant on every transaction. By our estimates, card transaction and execution fees constitute close to three-fourths of our $47.16 price estimate for American Express' stock. American Express also earns net interest income (net of the interest expense incurred on the funds raised for extending the aforementioned credit) on outstanding credit card balances, which constitutes about 6% of our price estimate. American Express' business largely depends on the number of cards in use, as this can often drive the number and aggregate size of transactions on which it earns a commission. Macroeconomic conditions such as employment levels and inflation rates (which impact the disposable income levels) can also influence average outstanding credit card balances. The number of credit cards in use in the U.S. has reportedly declined from over 70 million to roughly 62 million over the past year. We believe that most of this decline is likely not due to card cancellations, as most banks had already written-off bad credit card debts during the recessionary period between 2008 and 2009. We note that such write-offs were potentially less of a concern for America Express, given its more affluent customer base and conservatism in issuing cards (consistently resulting in the industry's lowest charge-off rates). However, voluntary aversion to using credit cards could be a serious concern for American Express. Here we explore how the decline in the number of credit cards in use could potentially affect American Express' stock value. We currently forecast the number of American Express cards in use in the U.S. will increase from nearly 30 million in 2009 to 34 million by 2012, eventually reaching 44 million by the end of 2017.