In testing the payment of several real accounts with actual payments due through the Elavon gateway, our customer found about a third of them failed. When they tried a few again through the module, they failed again. When they processed them manually on a credit card machine, they did not fail. The failure message they almost always received in response is “The field Customer Code (ssl_customer_code) required but not supplied in the authorization request." What do you make of this error? How do we resolve it?