Power Store will not be able to accomplish this.
You would be able to do this using eCart but it will require some custom coding to limit the number of uploads.
you would need to use ecart to offer the uploads for sale.
use Security assist to create the registration and login process.
Onthe eCart support page, see the tutorial for adding Security Assist registration and login to the eCart checkout process.
On the upload page, you will need to create a recordset that will be used to determine if the user is allowed to upload or not.
This is where the custom coding portion will come into place, we can offer a general overview, but is you need specific assistance getting it to work, I would suggest purchasing a Premier Support ticket where you can work one on one with an engineer to implement this.
The recordset will need to look in the orders table and the uploads table to determine a) How many uploads they purchased and b) how many uploads they have done. Only if they have available uploads, would you show the upload form.
I would create a table to capture the uploaded file name and the users ID:
uploadID - Primary Key
uploadUserID - foregn key, relates to users.userID
uploadFileName - file name
your orders table will have an ordersUserID column that will relate the order to the user,
your recordset would need to use a join query to link data from the upload table and the order table using the userID to see determine how many uploads where purchased, and how many uploads they have already done.
This part will be fairly complex, but if you have a good grasp of relational data and creating SQL statements to join two tables together and SQL Agragate functions like COUNT(), it should not be to difficult.