It did not charge a percentage fee to retailers , unlike Visa , MasterCard , and American Express .
Discover was also the only credit card accepted at Sam 's Club ; the retailer has since started to accept MasterCard and American Express ( see below ) .
Since it did not charge a percentage fee to retailers , unlike Visa , MasterCard , and American Express , Discover was also the only credit card accepted at Sam 's Club ; the retailer has since started to accept MasterCard and American Express ( see below ) .