This hook runs after all the automations and set the invoices to “Cancelled” it the services have been terminated or cancelled. This feature has been long requested to WHMCS but it was never implemented.
All you need to do to install and activate the hook is to upload the file into the /includes/hooks of you WHMCS installation. It will day after the cron runs it will set all the invoices to “Cancelled” if the related product is Terminated or Cancelled.
It will also create an entry in the client log to have a record or the cancelled invoice. The same log will appear also in the System Activity Log.
It’s as easy as that.