Used to be that eBay was very productive for our inventory that had not sold. Lately though hardly any sales of any substance. We used to buy boxes from Uline but their shipping costs were/are way too high. If I order over $100 from Staples shipping is free.
We ship lots of artwork and Priority mail is terrific. They did lose a $900 painting last year though.
http://www.thunderbirdfoundation.com/shop/
Have you used Amazon?