Opencart Shipping - Boxes and Packaging

Hi, We're currently using OpenCart to set up an online store for our trade customers. We've got most of our shipping costs and weight brackets sorted using ClearThinking's Category Based Shipping module. However, for one category of products we'd like an OpenCart module / modification that will accomplish the following: • If a customer orders a certain product, OpenCart will add another free product to the cart. This product cannot be removed from the cart. • The free product added to the cart will vary depending on the quantity of the product ordered. (for example, if the customer orders 1 - 3 of product A, they will receive product B. If they order 3 - 12 of product A, they will receive Product C (Product B will then be removed from the cart)) • If a product is removed from the cart, the free product will then be removed too. • The free products will have weights attached to them, and these weights will be used to calculate the shipping cost (using the Category Based Shipping module already in use) The reason we would like these free products to be added to the cart is that we package these products ourselves, and would like OpenCart to inform our packaging staff of the correct combination of boxes to use. In addition, packaging costs for these products is calculated using the cubic volume of the boxes they are packaged in, not the products themselves - and different products fit into different boxes to varying amounts - and as such the Category based shipping module is not sufficient alone to accomplish what we need. Apologies if anything is unclear, feel free to ask any questions and I'll be happy to elaborate. Thanks, Josiah