How to include packaging item cost in KIT item

Hello Experts,

We have a requirement related to KIT items which will be packed in a batch of 20 in a packaging box, my requirement is to include cost of packaging box item to KIT item.

One option I can think of to include the Package purchased item with 1/20 usage in BOM so that we will get the cost in KIT item but in some scenarios less that 20 quantities (e.g.15 quantity) can also be packed in a packaged box based on customer demand, in that scenario keeping packaging BOX item with fraction quantity in BOM will not provide the correct cost.