WHMCS already has an integration with the payment gateway: payment express . However this doesn't include the tokenized billing ability of this payment gateway. We need to have this module enhanced to support tokenization so that customers can have their card details stored and be automatically charged each billing cycle or for one off items. API details are here:
Our goal is to enable clients to pay for their initial purchase by entering their credit card details. Their credit card details are tokenized as part of this process. For any WHMCS billing operation including recurring billing, plan upgrades and one-off invoices which we wish to charge to their cards.