Hi
You can combine all PO relevant items into a single invoice based on certain criteria.
Please refer to the following links
http://saptechsolutions.com/pdf/VOFMCopyRequirementRoutines.pdf
http://saptechsolutions.com/pdf/VOFMDataTransferRoutines.pdf
http://saptechsolutions.com/pdf/SDBillingDocumentConsolidationAndSplit.pdf
Regards
Atul