I package the clothing item in these awesome newsprint bags. I bought a huge bulk package of them a few years ago, and that place has since gone out of business. I'll be so sad when I run out! I got that awesome Always the Forest stamp custom-made by http://www.etsy.com/shop/AsspocketProductions, and the other stamps are also from Etsy.
Invoice, business card, and a little note from me.
Maybe it seems like a lot of effort to stamp and sticker the bags and boxes, but it really doesn't take that long, and I think it's important for the package to be as nice as the clothing inside. Plus, stamping things is fun for me anyways!