how to set up service catalogues for rate based services

Summary:

Best practice for setting up and transacting against rate-based service catalogues where service quantities are unknown at the time of Purchase Order creation.

Content (please ensure you mask any confidential information):

Hi everyone,
We are seeking guidance on Oracle best practice configuration and operating models for managing rate-based service catalogues where the quantity of services is unknown at the time of raising a Purchase Order.

Scenario
We have multiple service categories (e.g. traffic management, legal services) where:

Suppliers provide pre-agreed rate cards (e.g. hourly or daily rates)
Services are delivered on demand / reactively
The total volume of work (hours/days) cannot be accurately determined upfront