Cristian,
Your transportation charge is at the header level and you want it to be applicable or not based on item category. Business could create an order with goods and service materials, in this case what will be the expected behavior - transportation charges applicable or not?
Does business create orders with only goods and different orders with service materials? Is the same order type used for both?
In my opinion, if you are looking into the logic of a header condition type, then you should base it on a header field, for example division VBAK-SPART etc.
TW