Discount for every 2nd or 3rd item in a category

Hi all,

I need a solution for a pecial case i have.
We sell shoes that can be customised by chosing heels, materials and sole-types.
I got every type of heel, material and sole categorised in hikshop, and priced them.

My problem is that it is not the materials or heels themselves that make the costs, but the job of customising is.

So, if someone for instance needs a pair of shoes with 2 custom materials, a pair of heels from stock, and leather soles, It will cost too much.
If a customer needs apair of shoes with only one custom material, or only the heelheight has to be altered then the custumer will pay a fair/normal prize.

My question is: is it possible to apply a discount for every 2nd 3rd etc.. product in a specific categorie?

Thanks for your attention!
Roy