Why do you want to post the components at all?
this would require an extra order for the components and a goods receipt,( if the PO is done correctly then you post the GR directly into vendor stock, if insufficient entered you even need to do an extra movement for that), and goods issues for the components.
All the costs for the components could be included into the service charge, no need to do all this overhead of work.
If all components are provided by you vendor, why do don't you just buy instead of ordering subcontract service?