Bear with me here, I have not yet set up a similair environment but this might get you started.

If you should be able to track the fee as a product (service, make to order) then your solution will work just fine. If this is not required, it might also work to configure your sales price based on the purchase price + 150 dollar. This can be done using the pricelist line (new price = base price (based on other pricelist) * 1 + -1 + 150). 

The item enters stock like any regular item, as a stock move. This will mean that in stock, all your items will be considered the same. For instance, if you sell watches and have multiple "seiko s1" watches, they will all be stacked under one product in Odoo. For this you can use the lot numbers to distinguish between individual watches while still having the benefit of stock value calculated correctly.

I would indeed create a PO for the items that you receive. If you were to use analytical accounting for this, you will be able to make sure some of the products in your stock belong to certain suppliers. That way, the appropriate partner can be assigned for the credit.

Anyway, this was all I could come up with. I hope this helps you at least a little and also hope that someone else on this board can provide a more in depth answer.