multi item shipping discount

im looking for a way to give a shipping discount if the customer buys more than one of the same product. i am using Per Weight Unit Shipping right now so i can define a certain amount for shipping an item. on some of my products i can fit more than one item in a box and save the customer some shipping and myself some lost sales.
some of my items are nuts and bolts that i have a $5 minim for shipping them. but the way its set up right now it will cost $15 for a customer to buy 3 bolts. is there a way i can set it up as follows:
1-5 bolts = $5 shipping
6-10 bolts = $10 shipping
and so on.......
if this can be done can i only do it to select products?

thanks

You will need to do quite a bit of customization to know when to use what count for what Product when there are multiple quantities in the cart ...