EDI output message for STO versus Drop Shipment

For IC AR posting (from the SD) we want the AP invoice to be posted automatically using EDI output message.
However, we need to differentiate between STO (in this case we need MIRO) and drop shipment (In this case we need FB60 since there is no PO)
MIRO/FB60 is driven by different output message in the billing document.
How it can be done if we want to avoid different IC billing type?