How to process Invoice & match with PO?

Content

Scenario: 
We have a contract with Supplier XYZ. Supplier is a broking company who is going to search best insurance policy for our Company. For this we'll pay broking charges which is fixed amount while Insurance policy amount is not fixed in the contract. 

Now, we are going to receive the insurance policy amount invoice from this supplier. 

Please let us know how to process this scenario in procurement/purchasing cloud module (should we need to enter BPA or CPA or new PO?) 

Summary
Supplier >> XYZ 
Amount for broking charges (Fixed amount) >> 10000 KWD ---PO Created
Insurance policy amount (Not Fixed amount) >> Not Fixed