Okay good, we're all on the same page! Problem is, my sizes would be much larger than standard mailers or sheets of cardboard.
The largest print I was going to ship is 12"x36", so I needed a package that was at least 14"x38" (since I wanted about an inch on each side) but couldn't find any stay flat mailers that were of such a large size. Plus, based on the fact that most of my works are long and skinny, I fear the longer it is, the more bendy!
So then I thought I'd just make my own. I purchased 26"x38" sheets of thick non bendable chipboard from chipboard distributor/manufacturer (this is harder than it sounds to find - looked up some local companies on Thomasnet) and then tape two pieces together at the appropriate cut size.
Good idea with the plastic cello bags....I was considering a thin bond or tracing paper, but plastic might be better.
Sigh. I'm going to experiment and report back.
Thanks for all your help everyone!