If one of your member's credit cards is declined, you may think that there is an error in your Checkout Process. However, if the Card Information is up to date and there are funds in the account, banks can still choose to decline unusual purchases.
Your first action should be to advise that the member contacts their bank or credit card company and requests that they let the purchase go through. After this has been completed, have your member attempt the purchase again.
Note: As a Site Owner, you have nothing to worry about. Your site is still live and your account is not in danger.