Hi Greg! Yes, that’s right, those are currently the two best options for dealing with a cost that only applies to one line item.
We do have the Entering Pricing for Line Items on a Purchase Order/Inventory Receipt feature request, regarding the ability to add the total cost charged for each line, and for Breww to then calculate the per UOM cost for you. I think that would help here, as you’d be able just to enter the total spent on the boxes, including the one-off setup cost, and Breww would then handle calculating the UOM cost.