It would be helpful to have some sort of validation step when calculating supplementary items. For example, if my order was just for item X, I calculate my supplementary item on header. It is added. If I add Item Y to the order at a later point, I then click calculate supplementary item on the header again, the results in adding the supplementary item for Item Y (as intended) but it also would add supplementary item AGAIN for item X (unintended).

I know you can calculate supplementary item on the line, but if you had a large order with dozens of lines, this would be a helpful validation step for a user.