freight calculation

I am giving freight charges in header level

i have 3 items

1. mat 1 in Tonn

2.mat 2 in kg

3.mat 3 in kg which is a free item

now the freight charge in the header is distributing in 3 items.

but I want then freight charge will not calculate on item 3 as it is free item.

So freight charge will distribute between item 1 & 2

How to do this?

Please help me