For Etsy selling, I find that I am most cost-effective when I'm buying shipping/packaging/art supplies on line. Here are a few of my favorite sites.
U-Line: they sell boxes and packaging materials of all sorts. I buy wonderful sturdy cardboard boxes (generally they cost between .35 and .65 cents each, for 7x7x7 squares and bigger!) made of recycled materials from them in bundles of 25 each...they don't mind small orders, they ship same-day, and you can call them 24 hours a day. Website:
http://www.uline.com/Bags Unlimited: great bags of every possible size, will ship small orders. Website:
http://www.bagsunlimited.com/Various art supplies: Jerry's Artarama. This site is AWESOME. Never buy anything at your local craft/art store until you check this site first. Prices are less than half what you pay at most retail stores...in some cases less than 1/3 ( I buy Liquitex soft-body acrylic paints from Jerry's for $3.99...same paint costs $12.99 at AC Moore). Worth the cost of shipping. Website:
http://www.jerrysartarama.com/Hope this helps a little.