How to create a packing slip?
Check out this list of order printing and order invoicing modules.

I think Commerce Billy looks promising, but I haven't tried it out.

Commerce Order2Pdf is another promising module, but I'm not sure how customizable it is.

I am currently building a custom module to handle my own packing slip requirement.  I'm building to a template, and have found FPDF to be quite useful for this.

If you're comfortable with custom module development, I'd think that FPDF is the way to go, but you'll just need to write a lot of PHP to build a template, and then use Drupal Commerce API and/or custom database queries to load in the order data.  If you don't like the sound of that check out the aforementioned modules.

Let us know what you end up using! I'm curious about those modules but in this case I am going the custom route.