Every one of my USPS labels is printed on the back side of recycled paper. I have my step-father Mark save all of his no longer needed work papers. Also, my printer is awesome! It is called a "brother" and I found it at Goodwill thrift shop for like $10! Sweet.
When I go to community rummage sales I always search for crafting supplies for my packaging including wrapping paper, ribbons, yarn, bags etc.
I do use the free shipping boxes and stickers from usps.com when I need to, however I've got my family and friends saving brown boxes and yellow padded envelopes for me. I would prefer to use re-used, sturdy, clean boxes I find from my neighborhood. I call it boxing for a cause.
Keep an eye out for free moving supplies on your local trade sites!
Save your kitchen boxes too to wrap goodies in (: