Payment Button Notification are used to increase security for your orders sent from "Pay Now" / "Subscribe" buttons that you are dinamically creating in your website. If you are using "Pay Now" / "Subscribe" buttons generated from our user interface you do not need to setup PBN.
The idea behind PBN is to sign the payment order with a key based on the submitted data and a shared secret. To activate PBN and generate a shared secret go to Merchant Services >> PBN Settings.
Creating PBN Signature