IDEAL
From WHMCS Documentation
Setup
To activate the iDEAL module in WHMCS, navigate to Setup > Payments > Payment Gateways and choose iDEAL from the Available Gateways dropdown. Then complete the fields that appear using details from the iDEAL control panel.
The certifcate related files (.cer and .pem) can be uploaded anywhere, we recommend above the public_html directory of your site, and then enter the path to those files in the Private Key and Private Cert fields.