Tried on Chromium, Vivaldi and Firefox and USPS Informed Delivery tosses a CORS error. This error does not occur when logging into the site via Android, OS/X, Windows. Had all extensions disabled on Chromium/Firefox and it still fails. Seems like a Linux thing.
They must of had something misconfigured. Just started working now but not an hour ago. I was starting to worry about my networking stack if something went sideways since it only happened on the linux box.