We are a trading company, dealing in Heavy equipment, in many cases our service Dept had to get outside repairs from local agencies, so I created a code "O/S supplies" service type, however If I invoiced this order no journals for the cost of this item

Example:

We have one sales order with some parts "Stockable products" and one line for "O/S Supplies"
I purchased this service with cost 1000 $.

How can I link the cost of this P.O into my S.O to have accurate profitability for this S.O.

I noticed that cost of any "Service Products" doesn't create journal entry on the Invoice!