How modify shopping cart ?

I’m really confused. I was asked to update a PHP web site and I don’t know PHP. Also, the site uses WordPress. My issue is that I need to add an item to the web site’s shopping cart.

* How do I find out what the shopping cart is called? In other words, what is the WordPress plugin to install? I saw several shopping carts when I did a search in WordPress.
* Do I need to install all of WordPress or just the shopping cart plug-in?
* How do I simply add another product to the shopping cart? I assumed that I would open a database and add it from there.
* Can this be done by someone who doesn’t know PHP and is not familiar with WordPress?

I’m overwhelmed by WordPress and don’t know where to start. Similarly, I can’t determine from the web site code how to track down the shopping cart code.

Web site: http://www.heartofkundalini.com/ebooks/

Thank you.