Use Case Services Already Rendered Bills

Request: What's the correct set of PeopleSoft "tools" and process flow for the following use case. We're billed for services that have already occurred, usually under an agreement (contract), with variable bills per month.

We currently

1. setup some sort of agreement (not in the system) with supplier to perform some task (e.g., laundry service, IT service)
2. as we receive each invoice, enter it into the system as a Requisition, send it through an approval process, expedite to a PO (which isn't really needed since we're not sending it), then create Voucher from PO.
3. nothing is done in the system to track the spending of this service against the agreement.