And without a doubt Occam's Razor here points to the USPS site having problems. Help from their tech support seems like it could be difficult to get without a pile of evidence so the next steps are going to be trying to nail down the outliers. If USPS tech support is needed we should make sure when you do reach out you're loaded for bear.
So, we know that your one computer is having this issue on each browser, and that it is happening with multiple network connections. It is either something on your computer closing the window for some reason or the USPS website is where the problem lies.
So, the next thing you would want to try is the instructions to clear caches on web browsers detailed here
If that doesn't work try again after disabling any antivirus you might have on temporarily,
Give it another shot after doing that. If that does not work I would recommend calling their Technical Support listed here USPS Contact Us page from the looks here it appears to be open again tomorrow there number is 1-800-344-7779
and the following are their hours.
Hours of Operation
Monday – Friday 8 AM – 8:30 PM ET
Saturday 8 AM – 6 PM ET
If they give you any guff or you want to spitball how to get them to help if they push back. Drop us a line in here in this thread, myself or another forum browser might be able to help navigate their tech support.