This is a nationwide issue that I've been following for the last couple months, but it does seem like the Detroit distribution center is a major bottleneck that's worse than other parts of the country. It's happening with both outbound and inbound packages. Based on articles I've read and from talking to postal employees, it seems to be a result of being short staffed both from insufficient funding and COVID (people are out sick or quarantined because of exposure). The area around Detroit was already having problems prior to the holidays, but the Christmas season really crushed it. When we moved here from PA this past summer, I immediately noticed that our mail was getting delivered really late in the day, usually around 5:30 or 6 PM. I'd never seen the mail come that late in the day in the dozen or so places I've lived in the last twenty years. As we got into fall, it started arriving even later, often around 8 PM as I was putting our youngest kid to bed (I'd hear the truck stop outside his window). On Halloween, the truck was making the rounds at about 7:30 PM while we were out trick-or-treating. I finally asked our mail carrier, who is a very nice, friendly woman. She said the carriers are all running two routes (which are supposed to be 8-hours each) because they're so short staffed. They deliver a route in the morning, and then they go back and pick up a second route in the afternoon and deliver into the evening. I saw this mentioned in a recent news article about the delays in the Detroit area. So that's part of what is contributing to the slow package delivery - issues with getting them out of the post offices and delivered, so the packages are backing up. I've had packages arrive at our local post office on Monday but not get delivered until Friday or Saturday. It used to be that if a package showed up overnight, it would be out that next morning for delivery.
The overall nationwide package delivery issue was described in a couple articles recently. A postal official said that they have had a 33% increase in the volume of packages since the 2019 holiday season. He said they planned for that increase over the next 3-5 years, but not for it to happen in 3-5 months, as it did toward the end of 2020. Apparently UPS and FedEx have partnerships with USPS to deliver packages (you've probably seen the ones where UPS transports a package to your local area, hands it off to USPS, and it gets delivered with the mail). Because of the load on those carriers, they stopped taking a lot of the packages from the USPS partnership right before the holidays, so USPS got that additional load back on them. I've heard that a couple USPS regional distribution centers, like one in central OH, have semi-trailers lined up in their lots for days waiting to be unloaded and sorted.
I've been selling a few things through eBay - not as a business, but just to get rid of some old collectibles and other things that we don't need, and to generate a little "hobby" money. I shipped two Priority Mail eBay packages from the Clinton Twp post office on Dec 7. The tracking showed that they left the post office that same day, there was an update on Dec 11 saying they were "In Transit - Arriving Late," and then... nothing for three weeks. One of them finally reappeared in Allen Park on Dec 26. It moved fairly quickly after that, arriving in Aurora, IL on Dec 27, went through two other locations, back to Aurora on Dec 28, and finally delivered later on Dec 28. The other package just reappeared on Dec 30 (23 days after mailing it) in Grand Rapids, and got to Nashville on Jan 1. I've decided not to sell anything else until after the holidays when this hopefully improves.
I've had the same thing happen with inbound packages. Another forum member shipped me a few magazines from IN via Priority Mail that were mailed Dec 15, arrived in Indianapolis on Dec 18, departed on Dec 24, finally showed up in Detroit on Dec 29, and got to me yesterday, Jan 2. One of the worst was a package that shipped from AZ on Nov 20, arrived in Detroit on Nov 24, and then it sat in limbo in Detroit until Dec 10. It was finally delivered to me on Dec 11.
They're also having serious issues with mail being directed to strange places. A friend in Maryland bought something through eBay from a seller who lived 30 miles away from him (also in MD). It shipped the next day, and a couple days later, the tracking showed it located in Florida.
I'm sure that was more than you all wanted to read, but it's a mess, and I wouldn't send anything through USPS right now that you want delivered in under two weeks.