They don't have them available at the post office. You have to order them online and they get delivered at no charge. I don't recall ever seeing them at any post office anywhere.
They still show up on the USPS flat rate page (probably because they'll still ship them), but looks like they're unavailable to be ordered. Maybe I'm just not looking hard enough? Can you post a link where you're able to order them? Those are a handy flat rate option for sure.