My post office says First Class is for PAPER items only, anything else is a package. I can usually get away with very thin cardboard and a few slot cards with the non-machinable marking. I was told if I marked the envelope DO NOT BEND then it would have to be mailed as PACKAGE. Furthermore, even if your post office lets you mail something First Class my post office would charge me postage due on your letter if they deem it to be a package. All bubble pack or padded envelopes, and even the stiff cardboard photo mailers all have to be mailed as packages.

If you want to save on packages you can try the pirateship.com website where you can get the maximum commercial rate discounts on all USPS packages, including some rates not available at the counter WITH NO MONTHLY FEES. Their site is totally free to use.