The best way to do this is to have the download page only show links for the products the customer has ordered.
eCart stores order information in the database using the store order Summary, and store Order Details server behavior.
On your Orders table, include a OrderUserID column to relate the order to the user.
This way the orders table will have Foriegn key relations to the OrderDetals and Users tables:
Orders.OrderID = OrderDetails.DetailOrderID
Orders.OrderUserID = Users.UserID
And the orderDetails table will have a foreign Key relationship to the Products Table:
OrderDetail.ProductID = products.ProductID
On the download page, you can create a recordset to show all of the downloads available to the user by filtering on the Logged in users session ID:
SELECT Orders.*, OrderDetails.*, Users.*
INNER JOIN OrderDetails ON Orders.OrderID = OrderDetails.DetailOrderID
INNER JOIN Users ON Orders.OrderUserID = Users.UserID
WHERE Orders.OrderUserID = $_SESSION['userIDSessionVariabel']
and create download links. This way if the user has not purchased anything, there will not be any download links.