530528 Nanacast customization

Hello, Please only respond if you have worked with nanacast before and know the system well. I'm looking to tweak their shopping cart so I can offer volume pricing discounts on multiple products. For example, let's say I have a catalog of 20 products ... I want the customer to be able to 'buy 3 and get 1 free' of different products that they choose. Here's an example from another site... [login to view URL] Thank you for your time.