USPS guidelines state that postcards need to be no larger than 4.25x6 inches, no smaller than 3.5x5 inches, no thicker than 0.016 inches. I've definitely sent slightly smaller and slightly larger postcards using a single postcard stamp, and I've received a couple of oversized postcards with postcard stamps. So, I'm wondering how closely most people hew to these guidelines. Do you make sure to use a first class stamp on larger postcards or do you send postcards of all sizes with postcard stamps?

I have a few really thick postcards right now that I find especially confusing...they are about 3 times thicker than normal postcards and very stiff.