Personally I feel you would be better of look at other modules for implementing this.
My initial thought is to put the documents in a Media Library and to link the individual document to a product page. This way you can manage the documents in a single place.
You could display the links by using a repeater web part and set the visibility based on whether the user has purchased the product or not.
This way it would be easier to manage since customers can use the Ecommerce module as designed (ie. to purchase). You could extend the model a bit by displaying purchased products on a page and thereby creating a link to the product info which in turn has links to the documents.