Let's take a look here and see if we can nail this oddity down. First, instead of the webaddress usps.com enter 18.104.22.168 to try and get to the USPS site. That is the actual address of the site which DNS systems translate usps.com into. If the problem is an issue with DNS you'll be able to get to the website. If not, we know a bit more by virtue of that test. If that still won't load trying via a VPN would be a good idea.
To address the concerns about VPN's, VPN's are themselves sort of a grey area as far as security, the safety of a VPN is determined largely by the provider. There are VPNs out there you can use for free and they are typically sketchy. There are those that you can pay for, of which some are great. You're basically connecting to the VPN provider's servers and surfing as if you were actually surfing from the provider's server. With Sonic's VPN which we provide to our customers at no additional charge is something we actively make sure is secure. So in essence, connecting to the Sonic VPN is something we'd consider safe where connecting to a VPN running out of an ex-Soviet country that lets you log in for free is not. VPNs done right are actually the only safe way to have employees connect to a company's network when they are working remotely as the traffic between the two is unreadable to outside observers.
The instructions here VPN Service
will walk you through how to connect. Connecting to the VPN and trying the problem website is a popular troubleshooting technique around here and would definitely help narrow down what is happening. If you can see the site over the VPN that will help narrowing down what is going on quite well.
All that being said, I have taken a quick look at your line, it looks fairly healthy. Try connecting to the USPS site while connected to the VPN and we'll be able to tell a bit more.