Attach ecommerce purch receipt to Paypal for links for downloads

I used Paypal “Add to Cart” buttons throughout my site. I also uploaded all my downloads into each Product Page. When I experimented by buying a couple of my products, I got a purchase receipt from Paypal listing the items I bought from my site with no download links. My entire site is downloadable products. How do I attach the e-commerce purchase receipt to Paypal so that my customers get that purchase receipt with the links to downloads instead of Paypal’s ordinary purchase receipt?

I searched for the answer here and couldn’t find a definitive description of what to do. Please help!