First: you must have time in your schedule to finish this project within two days. It is simple so that should be no problem provided you have the time in your schedule.
I have a custom .ASP shopping cart (not .NET) and I need to integrate PayPal API into the shopping cart. I will want the paypal process to proceed and confirm before the shopping cart stores the data in the database and send e-mails. All of this is already working on a couple of live carts with merchant accounts. I have just not integrated Paypal yet. The cart is designed to store a transaction ID returned by the merchant account for a successful purchase. I dont know what Paypal calls this ID but I assume it will be something similar. And you will be able to stick that in the database.