What happens if the SO needs to be delivered in two parts (something is back ordered and the client is invoiced for the first shipment, then needs a second invoice for a second shipment)? What happens if the client places 10 orders with you in a month and wants a single invoice at the end of the month? What happens if there is a mistake on the order (or the invoice) and you need to cancel the invoice you sent them and send a new one? Do these cases apply? Your request sounds like it would make easy of these examples very difficult to handle with the change you've suggested.