Print picking list before invoice

Hi,

I’ve spent literally days trying to work out how to do this (apparently) simply task. Unless we have a pick sheet there nothing for the warehouse team to pick from - at the moment we’re picking from the invoices but this means invoicing the order before i can print a thing. This seems insane as it’s perfectly possible that the invoice could change.

I want to be able to print the picking list first, but after trying pretty much every workaround going they all cause various php errors. I don’t want to install yet another extension to try and patch magento functionality, a simply patch would be preferable. I’ve edited the picking list template and got it looking the way i want it, but i can’t print the bugger!

Hope somebody can help, i’m running version 1.7.

Many thanks
Charles