How To Purchase labor based services (qty driven) - best practice?

Summary:

What is the best practice in Fusion to accomplish the routine purchase of high-volume labor based services? Is there a resource or documentation that could help us establish the right fundamentals in procurement process? What I have read about Fusion Services, PIM, etc. all seem to focus on selling services, OSP in Manufacturing, etc. and not simply purchasing labor based services from a 3rd party vendor.

The current solution in Oracle of using Rate Based Services to create purchase orders is taxing on our buying team. They process an exorbitant number of service purchase orders on a regular basis. The service quantities change based on location and project requirements, so they are not truly recurring in nature but rather routine.