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.
After you log into your ShipStation account, go to your settings, then choose packing slip
Choose to edit the packing slip. If you only have a default packing slip, you'll need to make a copy and then edit the copy. The default does not allow edits.
Choose the Footer section and scroll to the very bottom. Change the line that reads barcode to point to the order # instead. This can easily be done by highlighting "[Barcode]" and then choosing Order # in the right sidebar list of options. If your template does not have the barcode at all, copy/paste the following into the last line of the footer section template:
<p align="center"><barcode size="x-small">[Order #]</barcode></p>
The size of the barcode can be changed by changing "x-small" to a different value.
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 email@example.com on how to have this added to your account.