Invoice Service (duration) / integration with Shopfloor / Discrete Mfg

Hello,

I am looking for some ideas on how to implement following business case in Oracle R12 with Order Management:

We have a discrete Mfg processes and usually we sell these items "manufactured" in shopfloor management as ATO items (goods). Now I have a special case for which I want to invoice only the service of manufacturing, but ship the goods of course. The amount to be invoiced is based on the time/duration in the manufacturing process (machine hours or something similar).

In best case: I have one order line. then I have one work order in WIP / shopfloor management to "manufacture" something. I keep track of time needed for this and keep track of quantities produces. Then I ship the goods and invoice the duration.