Plesk Module is shipped with Plesk Addons - configurable Addon module that allows your customer to extend their subscriptions with Addons you offer.
Once you've configured your Plesk packages you can proceed to configuring addons. Make sure that you've created your addons in your Plesk in Plesk Admin → Service Plans → Hosting Plans → Add an Addon
Repeat steps 2-6 for any addons you wish to offer for your customers.