If you are a Shipstation user, you would print the Packing slips from Shipstation.
Shipstation allows you to add a barcode to the Pick List/slip.
It is key that the content of the barcode is the Order Number. Shipstation calls this the [Order #] field.
If you are not using Shipstation, Finale Inventory has a Packing Slip template that can have a barcode of the order number added to it for scanning with the mobile barcode scanner. Contact firstname.lastname@example.org on how to have this added to your account.