Somewhat complex shipping system

This post may be rather redundant, but.... My supplier is basically a network of several different suppliers. Shipping is as follows. Each item has a fixed shipping cost, if that one item is purchased, the shipping price displayed is the shipping that I'd pay. However, if more than one product is added from the same supplier, the cost of shipping is reduced. My shipping "table" is as follows:

If the shipping price is under $12.00 then...
The 1st product will have 100% of the listed shipping cost.
The 2nd product will have 50% of the listed shipping cost.
The 3rd product will have 25% of the listed shipping cost.
Anything under that will have 20% shipping.
If the shipping price on the first item is more than $12.00, then I pay full shipping price on each item.

Is there a way to program this into Zen-Cart? Because unfortuneatly, the shipping isn't a fixed cost/unit-of-weight ratio.

Thanks

- Steve