Adjustment made for the weight of the packaging to determine the weight of the goods (4)
I believe the answer is:
tare
(Other definitions for tare that I've seen before include "Weedy annual grass, often occurs in grainfields" , "Allowance for package weight; vetch" , "Allowance made for the weight of the container in which goods are packed" , "Rate a vetch" , "Vetch plant" .)