sandwiched between something that wont easily bend. Ive gotten art shipped to me in between masonite, thick foam core,and also cardboard & then boxed.
Next trash day, keep your eyes peeled for a plasma TV box. That is some thick cardboard and free. Some cheapskate dealers will actually get 100% shipping supplies from usps, Yes a multitude of swiped boxes for my $1000+ art leaves such an image.
The best one I've ever received was a commission piece form an artist sandwiched between two torn out sketch book covers, with a piece of cut hardwood flooring taped to one side, then placed in an envelope. His art rep (who is a member on these boards) would probably be mortified.
Maybe his tactic was to get you to buy enough art that you could re-do your living room floor?