2CheckOut

To setup the callback process for 2CheckOut to automatically mark invoices paid, go to the Look & Feel section of your 2CheckOut account and set "Direct Return" to Yes and the "Approved URL" to http://www.yourdomain.com/whmcs/modules/gateways/callback/2checkout.php

For added security, on the same page you can set a "Secret Word". If you set this, in the payment gateway configuration page of your WHMCS system, enter the same secret word so that WHMCS can verify that the callbacks it receives are coming from 2CheckOut.