Freight apportionment in shipment costing

HI We use shipment costing for freight calculations for stock transfers. One scenario is the shipment cost is a fixed amount based on route. What is happening in the system is that if there are multiple materials in the shipment, entire shipment cost amount is getting loaded onto the last material in the shipment instead of loading proportionately across all materials in the shipment.

Has anyone encountered this and how has this been resolved?

Sriram