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My customer called and said the site would not accept her Credit card  information.
when she  tried three times and it kept giving her an error: Payment declined please try a different Card.  I know it is not my credit card handling company ( paypal) because it never makes it to them.  So the Checkout module is rejecting the payment.  It shows the order as abandoned on the site with all her information  but she swears she entered it correctly .  
How do I determine what caused this problem?