Link between Inbound Delivery and Sales Order for Third Party

Greetings Gurus,

My client is using a third party business model in a retail setting that fulfills sales order via purchase req --> PO --> Inbound delivery with GR posting --> customer billing document created through batch based on the delviery quantity ('G' in item billing relavence) --> vendor invoice reciept happens subsquently.

My question is this, is there an inexpensive (performance wise) way to link the customer billing and the inbound delivery?

Since the inbound is not in the doc flow i don't think VBFA will suffice. I think the best route would be via schedule line back up, but i have yet to find a link.

There are process implications for this link for reporting and customer service which are a critical to the business. Point will be rewarded for sound advice.

Thanks for your help.