American Express does not check for CVV codes by default. Merchants who use the Kosmospay system to go through the Mercury Gateway in xcart must sign up for the CVV checking service directly through AMEX or opt out of CVV checking in the back of x-cart to be able to process American Express cards.
The following is from the pdf listed below (found on the American Express web site):
"Note the Card Identification Number (CID).
Your merchant account must be certified to
verify the CID number. To inquire more
about the program, send an e-mail to
es.fraud.prevention@aexp.com."
More info can be found here:
https://www209.americanexpress.com/merchant/singlevoice/USEng/FrontServlet?request_type=navigate&page=fraudPreventionBrochures
Specifically on this brocure:
https://www209.americanexpress.com/merchant/singlevoice/resources/CNPX%20Broch_032305.pdf
X-cart customers who choose not to verify CVV (called CID above) codes through AMEX, and who use the Mercury gateway and the Kosmos Payment system, must choose "CVV Not Processed (AMEX Cards)" in the "Charge order if the result of CVV check-up is:" field in the back end of xcart.
To get to the page containing this field:
1) Log into the administrator section of xcart.
2) Click on the "Payment Methods" link in the "Administration" sidebar menu, or go to:
https://www.YOURSITE.com/admin/payment_methods.php
3) Select the link called "Configure" after the phrase "Credit Card processor Kosmospay"
4) Highlight "CVV Not Processed (AMEX Cards)" in the "Charge order if the result of CVV check-up is:" field.
If you are unsure about any of this information, or if you have decided to sign up for CID (aka CVV) verification through American Express, please contact us.