in the webassist/plugins/shopping_cart/gateways/authnet.php file, find the following code:
$AuthNet_optional[0][$nextIndex] = "x_email_customer";
$AuthNet_optional[1][$nextIndex] = "TRUE";
and change it to :
$AuthNet_optional[0][$nextIndex] = "x_email_customer";
$AuthNet_optional[1][$nextIndex] = "FALSE";