Sellerboard users, how do you bake in inventory prep fees into the software/P&L?

Hello all,
Sellerboard has been great for me thus far, but I have not found a way to add in product-level prep costs yet. I pay a small prep cost for every unit that is prepped for FBA, that pep cost gets charged to me monthly, but I want to have it added into seller board on a product level, similar to how product shipping costs are baked in,
Thus far I have not found a way to do this. Has anyone here found a way to add an addition $1 to $2 prep fee per unit into Sellerboard on a product level?