This page can be accessed by going to System > Settings > Checkout
Go to System → Settings → Checkout
Invoices numbers generated manually by seller the store owner (administrator) on the order page.
Do not confuse Order ID with Invoice ID (number).The The difference between a sales order and a sales invoice is that a sales order is initiated by a customer who is electing to purchase something from the business and a sales invoice is the document a company uses to bill the customer for that purchase. A sales invoice isn’t initiated until a sales order has been processed.
Invoice Start No
Set the starting number the invoices will begin from. The next invoice you create will have the new invoice number.
Set the invoice prefix e.g. IN#. Invoice ID's will start at IN#1 for each unique prefix.