Sorry for the late answer, but I was not notified about your post.
The plugins is only to interact with the workshop, e.g. Upload/Download/Query/Item Stats/… I did not yet get into paid workshop items. In case a workshop item is paid, I gues the user has to buy it first, otherwise the Workshop API (which is what this plugin connects to) should return an error when the user tries to download an item he did not buy.