I would like to optimize shipment sizes by taking both shipment and storage
costs into account.

I would like to use the goal seek function to minimize the value in a
certain cell (total cost) by changing another the value in another cell
(shipment size).

However, I have found that the goal seek function is set up in such a way so
that the value in the goal seek cell is set to a certain value and that there
is no way to minimize that value. Did I miss anything? Or I should just set
value to zero to minimize it.