to make modifications to the shipping in PowerStore, you need to use our eCart extension.
using eCart, you can remove the existing shipping rules from the eCart object and add a new rule.
in the new shipping rule, use the trigger Number of Unique Items > 0
Set the calculation to Based on multiple of column subtotal:
SubTotal of column "Quantity" times "4" "plus" "0"
change 4 above to amount you want to charge for each item.