For the most part, Reptoid has it right here. As a franchise multi-dealership parts manager I deal with this stuff everyday. The only real USPS issues (delays) I've been seeing stem directly from the shipper. This goes for UPS and all the rest. And in defense of the shippers, supply issues have been a problem for a lot of them causing delays because of (their suppliers) company cutbacks stemming directly from the COVID crap our "leaders" are shoving down our throats. My job has become a literal fight everyday simply trying to satisfy customer orders because of all this. Before any of it has a chance of getting better our countrys leaders, at all levels, have to get their s--t together and allow business to make moves toward returning to the old normal. Til then it is what it is and we're all being forced to swallow it.
What Wayne said, at least 95%.

I've had USPS take the long way cross-country, but at least they usually get there. I ordered landing gear from Hobby King and believe they arrived - from China = in about 5 business days or less. Somebody's doing a pretty good job!
Re Amazon: I believe putting the Postal Service out of business is the very last thing they want to do. Many of us could become Amazon Sellers - they ship products from about a zillion points and get it, in many cases, to the Postal Service for residential delivery. And UPS charges more for delivery to residential addresses than to commercial addresses.
It makes sense: we order one of these and two of that, while a business might order by the case or gross. Letting the Postal Service make the final delivery is very cost-effective. If you receive prescriptions by mail, there's a very good chance that's exactly what's happening.