WHMCS is a PHP app that provides a customer-facing billing and support system for hosting companies.
The name WHMCS stands for WHM Complete Solution. So what's WHM? WHM, or WebHost Manager, is part of cPanel's hosting control panel. As WHMCS was originally created in 2006 to add a billing system on top of cPanel, they used WHM in the name.
Since 2006, WHMCS has become an extensible billing and support system on top of many hosting control panels, not just cPanel. This support for many control panels is done through WHMCS Addons.
There are currently no actively maintained WHMCS modules for selling hosting using ServerPilot.
If you prefer not to use WHMCS, the following WooCommerce module exists for selling DigitalOcean droplets managed with ServerPilot.
First Release: October 2017
WooCommerce Hosting allows you to sell and configure droplets. The plugin supports DigitalOcean and ServerPilot.