Ideas on rating based on cost line

I need an accesorial cost to take the total cost of the shipment and charge a % against it. But if there is a special accessorial i do not want it to include that cost.
So..
Base = 100
Fuel Accessorial = 100
HST Accessorial = 50 (bad accessorial I do not want to include)
GST Accesorial = (100+100 time 4%) Base+Fuel total cost do not include that HST

Any thoughts??