WHMCS

Web Hosting Billing & Automation Platform

https://www.whmcs.com/

Main features:
Gateway modules (Receive payment via paypal/stripe/...)
Provisioning modules (Automatically set-up user's service. e.g. webhosting(Cpanel, directadmin, ...)/vps (Esxi, hypervm, ...)/...)
Registrar modules (Automatically register a domain name on behalf of the users different Apis)
Notification module (Renew reminder, marketing, etc via Email, Sms, ...)

WHMCS allows users to [create their own](https://developers.whmcs.com/modules/) custom modules to extend the functionality or add new services/payment gateways/registrars/notification platforms.