in the eCart object, add a column for itemShipping. on the calculations tab add a new calculation as:
[itemShipping] * [Quantity]
on the shipping tab, create a new shipping rule using the following trigger:
"Total number of Unique Items In the cart"
if the total number Of unique items in the cart is ">" a value of "0"
and set the calculation to:
"Based on multiple of column subtotal"
Subtotal of column "TotalPrice" "Times" "1" "Plus" "0"
in the add to cart button behavior, bind the itemShipping cart column to the shipping column in your products table.