Some customers and indeed shop owners may need to pay for goods and services by Invoice. If you or your customers need this option, then you can add this to the Checkout as a new Payment Type.
Within your Shop's Admin area navigate to Settings > Payments and select the "Purchase Order" under "Manual Payments".
Add a relevant checkout message for the customer (who will see this on their order emails and at checkout) to inform them of where the payment needs to be sent, any payment time limits, etc.
Set the Status as "Enabled" and then hit Save.
Customers can add their Purchase Order Number at the checkout.
This will be added to both the Purchase Order and Invoice that you can print from the Order Screen.
Once funds have been received and confirmed then you can continue to mark the order as paid and shipped as normal.