Hi,
Standard SAP will determine the document type assigned for ME21N or ME21 assigned in OLME - Define Default Values for Document Type; while creating shipment cost document.
If you want a different document type, then you have to go for development using the enhancement: V54U0006 in EXIT_RV54POCR_006. Refer the note: 506605 - Info: PO generation when transferring shipment costs
Regards,
AKPT