The "Paid" designation just indicates that it is dependent upon a ServiceNow Product that requires a subscription. So for example a plugin may require a subscription to Customer Service Management. In this case Allen is explaining that On-Call Scheduling requires the Paid subscription to IT Service Management. This is definitely a frustrating aspect as it can be difficult at time to know exactly what the required prerequisites (in terms of subscriptions are) for a plugin to be available. Hope that helps.
