I reduce and reuse.
I stopped printing out the invoice to include in the packaging. I realized that it's just a waste of paper, and when I'm buying stuff, I don't care for it. So I figured buyers won't care for it either, and if they need it, they can find it in their Etsy account.
I also try to reuse packaging materials. For example, if I receive a packaged gift with nice packaging paper or ribbons that are in great condition and can be reused, I'll definitely save them for my future packaging.