I’m not sure I understand what is required here. I thought Manager already supports this.
If you issue delivery notes without issuing invoices as is the case here, then under Customers tab, there will be column Qty to invoice. This represents items which have been shipped to customer without being covered by sales invoices. When you click on the quantity to invoice for the customer, there will be New Sales Invoice button which will copy all uninvoiced items from delivery notes onto new invoice.
I’m removing this from ideas as my understanding is that Manager already supports this unless I’m misunderstanding something.