Except for bailouts - like the one currently being proposed - USPS is self-funded and does not receive money from taxes. One problem is that USPS is meant to be a service (congress has to approve rate increases), but it's treated like a business. This wouldn't be such an issue if people weren't trying to sabotage it so that they could privatize it.
I've heard the deal with Amazon is bad, and the new Postmaster General is trying to renegotiate it.
In my experience, no Priority or First Class packages were delayed in favor of Amazon packages, but the flood of Amazon packages did overburden the system causing some delays. From what I understand, USPS is receiving more Amazon, because more people are shopping online (Covid) and because Amazon is (was?) dealing with a labor dispute. A couple weeks back my station got absolutely hammered - received about triple the typical Amazon volume on back-to-back days. The clerks have to sort each package to the appropriate routes, and then the carriers have to deliver all those packages. As they're out delivering, they're picking up packages to bring back to the station, and those are the packages that were subject to delays.
In general, everything goes out every day, and even if one package is left behind, someone has to drive that package to the plant. From a business standpoint, this doesn't make a lot of sense, but it's part of the service culture at USPS, and clerks and carriers are horrified and mortified to hold up mail, which is why you see complaints like the one from the carrier in the article.
At the same time, USPS was rolling out some new policies, and they're missing a lot of people due to Covid, so a lot of factors converged to make the situation difficult. But, at my station, no mail has been delayed for weeks. I can't speak for the plants, but the vast majority of Amazon packages are delivered to the stations as drop shipments, so if the plants are having issues with volume, it's not because of Amazon.
During Covid, letters and flats had been down in volume (businesses being closed = less advertising) by somewhere between 25% and 40%, but they appear to be back to normal, and parcels had been up something like 60% overall.
I think that's most of the pertinent information. But, I'll say that this is typically the time of year where many USPS employees exhale a bit. The overtime dwindles, and the pace slows. Instead, much of this has felt like Christmas - anyone on the OT list and any non-career employees are working lots of hours to keep up with demand, and sometimes they're even compelling people not on the OT list to work OT. So, it feels like the holiday season never stopped, and we're all pretty exhausted. Of course, we feel lucky to have been able to work through the pandemic, but it's taken its toll, as well.