To start with, the user will need to register and login prior to purchasing, see the following tutorial for details:
when an order is placed, the users ID should be store in the orders table. The order details table will have the order ID stored. Using a jpoin query, you lookup the orders to tell how many uploads the user has purchased.
When a file is uploaded, you should have another table for userUplaods:
useruploadID - primary key
useruploadUserID - relates to users .userID
useruploadFileName - stores the uploaded file name
on the upload page, you will need to add logic to lookup the number of uploads purchased and the number of uploads completed and restrict access if completed is equal to or above purchased.
if you need help implementing this we can help in a premiere support appointment: