I think that it's worth the price to pay a little extra for the peace of mind that the heavier cardboard & mailers provide. It's why you charge for S&H, after all, and your customers will appreciate the effort.
I put my prints in a manila envelope, then sandwich it between 2 heavy pieces of cardboard, tape it together, then I put that inside a rigid mailer. The 9x12 rigid mailers cost about $2.40 to ship out + approximately .80 per mailer, but I'd rather be safe than sorry.
So far I haven't looked into pre-cut cardboard, as it turns out I have enough lying around to last me a while :D