packaging materials (roxbury)

I am a starting out trying to open up a brand new thift store online. I have made a couple of sales but the price for packaging at the post office is killing me. if anyone knows of how I could get a constant supply of free already used boxes that I can recycle I would be eternally grateful. I have social anxiety so that is why I am trying to do this all online. Also I only have a push cart and I don't drive.