An invoice is a shipment in Sage 100.  Why do they want the "invoice"?

We have created a custom crystal report that mimics our invoice as a ProForma invoice.  We use that for international shipping.  But, we print this after the shipment is already created and prior to posting the invoice, in order to get as close to accurate information as possible.  Maybe the same concept can be used.  

I've done the same type of thing with a customized SO form code.

We have modified a sales order form to be a "ProForma" invoice that we can send to customers who need an invoice so they can pay in advance of shipment.