It's mentioned on the page you linked to:
OTRS Feature Add-Ons are useful additional features that make your OTRS more flexible and tailored to your needs. To show that we appreciate your trust in our professional services, Service Contract customers get exclusive access to these additional features and receive free upgrades for any purchased modules during a current major release cycle. As a Service Contract customer, you can choose from the following OTRS Feature Add-Ons free of charge, within the limits of their compatibility. We are happy to consult you on that matter.
But I said something wrong: As a customer you can get the add-ons free of charge.