Hi,

The easiest way without any programming would be to create a separate product for each products package and sell it as a stand alone product (SKU).

Otherwise you will need to change the shopping cart controls and in the first step you will need to check the content of the shopping cart and add/remove products according to your rules. Here you can find some information about how to customize the checkout process - http://devnet.kentico.com/docs/ecommerceguide/customizing_the_purchase_process.htm. So, you can find appropriate controls in the process and change their behavior according your needs.

Or, as I mentioned before, this option will be in upcoming version 6.0 which is planned to be released within 2-3 months.

Best regards,
Juraj Ondrus