I would also like to understand how to programmatically create pack slips for multiple orders to be sent to a 3rd party vendor who will be fulfilling those from a different warehouse then the majority of our products.
@Andy_Hobday1, we've had a few headaches with packing slips too. We were first using the shopify default, but then switched to Apps to be able to print them in bulk.
Currently we're using personal print for our invoices because it lets us smartly personalize our invoices for each customer, and they can then be downloaded directly from the Shopify orders' list page.
It's incredibly the only App we've found with a drag and drop builder that truly let us create what we wanted...
We're also using it to send some automated personalized thank you notes to our customers for about a year, and we love it for that!