Recently we have released a new plugin - BitcartCC Checkout for WHMCS.
All you need is your BitcartCC instance.
To install the plugin, copy
modules/gateways folder in our repository to your whmcs
Source code: https://github.com/bitcartcc/whmcs-plugin
You will need to provide your instance's admin panel URL (for checkout modal), and API URL (to create an invoice).