Currently, permissions/access to items are handled by groups.
With the exception of the Downloads application, where a user can pay to download a specific file/item, most purchases/subscriptions in Invision Community focus around the ability to gain access to X categorization of content/area/etc...
Good example in terms of courses might be a Beginner, Intermedia, and Advanced subscription level and then an All level. That's 4 different groups which would gain access to either segments of content or all the content. It is not really advisable to do 1:1 relationships as that can create an unnecessary amount of groups.