Is there any billing system included in a Plesk installation package?
Does Plesk offer a host management / billing front-end like WHMCS?
There is no billing system shipped with a Plesk installation package. It should be installed separately.
Plesk can be managed via API requests and therefore can be integrated with any billing system that is able to communicate by API requests.
Plesk has an integration with billing software such as WHMCS, HostBill, BILLmanager and Blesta. For more information, visit official websites of these software providers.