Built and sold by whmcsmod
The integration is an add-on module for WHMCS, made by whmcsmod, an independent WHMCS module developer. It's a paid module: you buy it from whmcsmod, which sets the price, and install it in your WHMCS admin. ChartMogul doesn't charge anything on top of your ChartMogul plan.
How it works
Once it's installed, the module sends customer changes to ChartMogul, and pushes paid and refunded invoices in real time. Cancelled and terminated services update your MRR. Scheduled tasks keep everything in sync, and bulk exports bring in your existing customers, plans, and invoices.
Getting it
The module is listed on whmcsmod's site and the WHMCS Marketplace. whmcsmod's own documentation covers installing and configuring it.