Invoice is a payment method that lets you send invoices to your customer. You can enable this under Settings > Payments > Invoice.
How to use Invoice feature:
Choose the products you want to sell
Choose invoice as a payment method for your sale, the invoice window will now appear
Search for an existing customer or add a new one
To add a new one, fill out the basic details. If the customer is a business you can fill some extra data
Issue the invoice
Do you want to use this method without actually sending an invoice?
Go to Settings > Payment and enable Pay on Account