Thanks for the explanation.
In this case, on the check out success page, you will need to add a record set that pulls the order information from the orders table and make sure to match the order id in the cart with the order id in the table. (note: you will need to do this at the top of the check out success page, because the session is cleared at the bottom). And once you have a valid record from the database then you can update the flag in the subscriber file.