Hi,
As you already said for service PO- you cannot have delivery condition type where service is performed by a service vendor but in your case you have commission vendor charged a commission for arranging service from your service vendor.
Better create two PO's
1. One PO for your service vendor
2. Another PO for your commission vendor
OR
Create one service PO for service vendor and for commission vendor create invoice in FB60 after invoice verification completed (MIRO) for service PO.
Regards,
Biju K