Thanks Jason,
I just got answer from the PayPal, and they said we will get notification of orders and they send receipt to the customers without IPN.
We just want to keep it simple and I don't think we need it as long as they notify us anyway. It sounds ok?
We also bought premium ticket to customize the add to cart button function to connect with product info of our sql database. I don't think it has nothing to do with the IPN?