[Ed B.]

I would take a look into /skins/admin/order/packing_slip/parts/body/items.twig classes/XLite/View/PackingSlip.php (these are the files responsible for outputting packing slips) and classes/Model/Repo/Order.php, classes/Model/Repo/Product.php (these are the places where the details of sorting are defined), and maybe classes/Model/Order.php classes/Model/Product.php (these classes should contain sorting methods). Well, I know, this is kind of lots of places to look...