act as a shopping cart handling all these items?
No. CBSubs does not provide a shopping cart fimilar to that of sites like Amazon, etc.. CBSubs is first and foremost a subscription management extension. While "Merchandise" plans are capable of being created there is no quantity controls or shipping and handling type capabilities.
The best approach is to use CBSubs to handle your subscriptions and use Virtuemart to handle your products. You can use CBSubs to provide paid access to Docman where you'd distribute your digital goods.
Also, with regard to licensing, we have site users who register through community builder for a free account right now. Does the software distinguish between free users and paid users with regard to how many user license we need?
No, your CBSubs license is for users total; whether paid or free does not matter.