Add items to cart at checkout

I have seen stores on other sites include a "freebee" item at checkout. My client would like to do the same with some restrictions.

ex. Customer buys (2) ProdA and gets (1) ProdB free.

I am using an extension Sales Promotion (VQmod) that will refund the cost of ProdB - IF IT IS IN THE CART! My problem is getting the item into the cart without the customer having to do anything. Too many customers complain that they expected the store to give them the item automatically.

Does anyone have any examples of code that can be placed in the Cart and/or Checkout pages that will review the items in the cart, apply a few rules and add a product to the cart? I am just staring with customizing Opencart and need some pointers in the right direction.

(Also, suggested reading for a beginning Opencart coder to understand the basics)

Jon Black
for
Syncom Pharmaceuticals, Inc.
www.syncom.net