I think you are misunderstanding how the download center page is created.
the products table will contain information about the product onluy, it will not copntain the userID column. The products table should contain the productID, productName, description, downLoadPath, and digital or physical columns.
before the user can checkout, they need to login, which will set the userID session variable, this is the only session that is needed.
In the checkout, the cart will store the order summary and order details.
the order table should have a column for orderUserID, the store order summary behavior will set the orderUserID column to use the UserID session variable value to associate the order to the user.
On the download center page, a join query is created that joins the order, orderdetails and products table based on the orderUserID column and the user ID session variable to lookup the downLoadPath for digital products.
For more step by step instructions, see the "Applying Register/Login to your checkout", "Automatically populating checkout fields", "Creating an Order History page" and "Creating a Digital Goods Download Center" tutorials on the eCart support page: