WP eCommerce plugin 3.7.6.5

Our shop site uses the WP ecommerce plugin (version 3.7.6.5)

We're doing a promotional offer, whereby customers get a free item if they buy 3 units from a specified product category. Once the shopping cart has at least three units, the free item should go into the cart automatically, or a message should display that the free item will be added with shipping.

Or, the customer should automatically get a 25% discount if he has four units from a designated product category.