You can print the invoice for an order under: eShop -> Orders -> Print.
You can edit the layout of the print view under Settings -> Advanced Settings
-> Text modules 2 -> Texts for the printed view of the invoice
You may require other settings than those on the standard service pages for the
print overview (e.g. good font colour contrasts). You can edit print_order.shopscript file.
eShop -> Shop design -> Edit service page templates -> Service page:
Print view of the invoice (print_order.shopscript)