I'm looking for a php shopping cart that can handle a particular, non-typical pricing workflow. I'm selling an item with additionally priced options, up to the first 3 of which are included in the base price.
For example, I have a $100 base product that can be ordered with around 40 additional options. Each option costs $10, but up to 3 options are included free with the order. So, The base product with 1, 2 or 3 options is $100, but the base product with 4 options is $110; the base product with 5 options is $120; etc.
Is there a downloadable (non-hosted) php shopping cart -- free or at any price -- that will provide this functionality? Are there a few?