How to get shipment lines with prepayment

belindasphoto

Users want to use sales invoice to combine shipments from different orders (by using the "Get Shipment Lines.." function in sales invoice subform). Meanwhile, there will be lots of prepayment cases.
Here is the problem, sales invoice is not designed to get shipment lines with prepayment.
In this case, how could I combine multiple shipments with prepayment into one invoice? Could anyone help? Thanks very much.