Automatically sets the invoice number as soon as an order as it's placed with a complete, processing status or always.

Install extension via Extension Installer, then install the Module: 'Order - Invoice Number Auto' selected the relevant complete order status. This works by adding a new event: 'order_invoice_number_auto' to be called before: catalog/model/checkout/order/addOrderHistory/before.

Also the 'Invoice Prefix' can be updated automatically based on the date order is placed. E.g from store settings set value 'INV-{YEAR}-{MONTH}-' which will generate invoice number: INV-2016-12-11