Not sure where you're finding them online, but it looks like Rob was right and that they've been discontinued.
Discontinuance of the Priority Mail Flat Rate Large Board Game Box | PostalPro
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.
Priority Mail | USPS
