Item-wise Purchase Charges

I have a purchase tax and charges template of type “Actual”. The amount varies from one item to another. I cannot set the item wise amount. The template is not getting listed in the item’s tax section since it is not a tax. How do we handle this?
Basically I need to configure item wise charges for packing, unloading, …

As a workaround, I have added a custom field “other_charges” in Purchase Order Item. This will include additional charges of each item. Now the problem is I’m updating the amount to amount + other charges in cutom script before_save. But the amount is set to default amount after save. How to resolve this issue?