not so much as bid. but i found your product over at twitter.
Anyway, i had an extension made up for me.
Allows me to create shipping categories. Each category gets assigned a price.
Eg. Category 1 - $10
Category 2 - $15
and so on.
You then assign a shipping category to a product, lets take an example product "TSHIRT"
this product is assigned category 1 (ie. $10) for shipping.
So if customer orders:
1 x TSHIRT - shipping stays $10
2 x TSHIRT - shipping still stays $10
*here is where it works)
3 x TSHIRT - shipping is now $20
Essentially it adds a shipping cost for every 3rd item of this SKU in the basket.
However, i have a flat cap (eg. $100) whereby shipping will NEVER exceed this value. Even if customer adds a hundred products into his basket, shipping remains at $100.
Obviously I have set it to add a new value at ever 3rd item. I'm certain this can be changed to every 5th item.
Lastly, i did not code this, i merely got a development company to do this for me. It cost me about $500. Happy to recommend them to you and they could modify my version at a lesser cost then i paid.