Visit the Site
Add Products to your Order
Proceed to Billing page
Populate all billing details and select Pay by Invoice
Enter a purchase order number (optional)
Press Submit Order
Important: Once an order is submitted, it remains in a status of "Payment Pending". While orders are in this status, they can be cancelled by the order administrator and the Products can be activated until payment is made.
Paying a Payment Pending Invoice
More information available here
Below is a example of checkout via invoice:
The checkout via invoice option will only appear on Sites that have this feature enabled.
Once enabled, it appears as text, under the Credit Card field.