This help page explains in detail how to set up PDF Stamping for your product:
http://www.e-junkie.com/ej/selling-ebooks.htm
When you add a new product or edit an existing product, you will see where to tick the box for Custom/Third-party Integration, which will reveal the Product Notification URL field where you'd paste the URL to enable Stamping for that product (which we provide and explain further at the link above).
