Plesk comes with a number of free site applications that you can choose to provide to your customers or remove from the control panel script library (referred to as application vault). You can also obtain and install commercially available applications from third-party vendors through the application portal www.apsstandard.com.
You can set usage rules for any site application you add to the vault. For example, you can make a certain free application a commercial one that will be available to your customers only if you add it to their application pools under your conditions (for instance, after you received payment).
Application packages upgrade is also supported. If new applicatinon version is available in Applicatin Vault, installed on the domain application can be upgraded by clicking the corresponding shortcut in the U column on the "Domains" -> "domain name" -> "Web Applications page".
License Keys for Applications is managed in license keys repository at the
"Server" -> "Application Vault" -> "Repository of License Keys" page in Plesk CP.
For more information see "Serving Your Customers" -> "Before You Start Serving Your Customers" -> "Prepackaging Applications Your Customers Can Use" section of the Plesk 8.3 administrator guide.