Pro forma invoices with regular order types

My client is looking for to send customers a prepay Invoice or a Proforma Invoice.  Basically Enter an SO and generate an invoice, and once payment arrives they will process shipment and invoice. I set them up using credit terms with prepayment required and it works great. My question is if there is a way to do proforma invoices? I’m thinking I can create a second version of the sales order acknowledgement form and make it look like an invoice. Any other ideas?