This will cause the Buy button and the View Shopping Cart link to show the shopping cart up in a popup.
- Hover over Shopping on the left-hand menu, then click on Setup: (Your cart name).
- Look under "Display options / Customise Text / Configuration" and click on "Category and product index pages".
- Using the dropdown next to "Buy Button Goes To", select "Popup".
Now your customers will have a more streamlined shopping experience.