CashU
From WHMCS Documentation
Activate the CashU module in WHMCS by navigating to Setup > Payment Gateways and enter your details as requested.
Then in your CashU control panel set the URLs as follows:
Notification URL: http://www.yourdomain.com/whmcs/modules/gateways/callback/cashu.php
Return URL and Sorry URL: http://www.yourdomain.com/whmcs/viewinvoice.php