Hi Guys,

My first post here and new to e-commerce, so please forgive me if some of the information seems a little vague or not too technical.

I am looking for a shopping cart that can add bonus quantity to an order based on how many of that item is ordered. I am trying to get Buy 3 get 1 free or buy 2 get 1 free. Many of the shopping carts I looked can do that but the wrinkle is that they usually do that based on a percentage discount. But that's not what I need. Maybe this table will explain what I need

Buy 3 Get 1 Free
-------------------
Order Extra Total
1 0 1
2 0 2
3 1 4
4 1 5
5 1 6
6 2 8
7 2 9
8 2 10
9 3 12
10 3 13


You can see the pattern here. So the question is: what shopping cart(s) will allow me to do this?
Any help will be appreciated.

Thanks in advance