This module hasn't been updated since the 1.3.5 release in 2016. I needed to fix it so it works with the current version of WHMCS and PHP.
The CloudFlare WHMCS Module allows hosting providers to offer CloudFlare through the use of Product Addons with any hosting plan sold. Hosting Providers choose the hosting plans these addons can be attached to and site owners (the end-customer) can choose these addons during initial checkout when they first order their hosting or afterward from the WHMCS Client area.
To use the module, the hosting provider must be a CloudFlare certified partner. Partnership is free and has no costs, risks, or commitments. To apply to be a CloudFlare certified partner, just visit: https://www.cloudflare.com/certified-partners
Link to installation and training doc https://www.cloudflare.com/media/downloads/Cloudflare-Training-WHMCS-Installation-Guide.pdf
Included in the repo for reference
Original BSD 3 license.