You can detail an order's inventory at any time after that order has been marked as Picked Up but before it has been marked Ready for Delivery, which initiates customer payment. To detail:
Find and click the order on the orders page and make sure that the order you want to detail has been marked as Picked Up has not yet been marked as Ready for Delivery (see Order Process for more information).
On the order modal, click on the Items tab.
Find the services you want to add to the customer's order under the appropriate categories.
For each service, either enter the quantity manually or click the + button to increase the quantity by one.
When you have completed detailing the order, click Save.