Upon order fulfillment, a printed invoice should be added to the inside of the box before shipment. This can be done directly from the Order page in your Maker Portal.
After adding the shipping cost, carrier, and tracking number, the order will show a status of "Complete." Find the Print Invoice (pdf) button on the right side of the order details page.
Alternatively, use the "gear shift" drop-down at the top-right of the page: