Services - Requisitions/Releases

I'm not sure my Subject line is accurate but here is the situation.  We are trying to figure out the best way to handle our "service" type requisitions/releases.  We recently just upgraded to the R12 instance and are looking at implementing iSupplier. We need to condition our suppliers to have PO's references on their invoices before sending into our AP department.  iSupplier they will be forced to enter their own invoices but only if their is a valid PO to issue an invoice against.

We are a utility and have several divisions utilizing the same supplier (contractor) to do work out in the field.  We have created blanket PO's so a release and a statement of work is required to be able to perfom work.  Currently our project managers are creating requisitions (releases) after the fact, once they know what the dollar value of the service is, they enter their requisition and route for the appropriate approval.  Can you recommend how your company is handling these types of services?