Our credit card plugin supports payment processing through Visa, Mastercard, Discover, and AMEX.  

If you are testing your installation in Live mode and receive a Payment Error: DECLINE message when you are certain the card information is valid, the most frequent culprit is mis-configuration of your Merchant Account at Vantiv (when all other card types tested work fine.)  For some reason, it seems that the AMEX application process is prone to occasional hiccups during registration and to a lessor extent Discover as well.

If you are certain that your account is configured for AMEX, please take a screenshot of the error and send us a ticket.  If possible, please include the URL of the browser in your screenshot. 

If your plugin is already configured and authenticating with Vantiv and you add in AMEX support to your ECOM account later, you will not need to update anything in the plugin configuration. The plugin simply ensures authentication with Vantiv from your account, and Vantiv will automatically approve the new transactions submitted with your merchant account without the need to change anything in the plugin configuration.