Randolph Hoppe:
Duh. I turned on logs and thought the result might help.
You're running 2017-09-16, which doesn't 'understand' the rebranding that USPS has done over the past couple of years. The current version (2022-08-07 K11e) might work for your woefully old Zen Cart base, but I promise nothing! As @dbltoe indicated previously, you'll need to record your current settings and remove the module via the admin's Modules->Shipping, upload the updated versions of /includes/languages/english/modules/shipping/usps.php and /includes/modules/shipping/usps.php and then head back to the admin's Modules->Shipping to re-install the shipping module with those settings you recorded previously.
Note that if you do install the K11e version, too, that you'll no longer receive notifications that a new update is available (if/when one is) since the processing that does this is built into zc152 and later.