I have previously worked for USPS and can help clear up what is happening. The mail carrier and their assigned station (Post Office) should be thought of only as the ingress/egress of the postal system. It would be wrong to say that no sorting happens there, but the type of sorting that doesn't happen there is why your mail is being postmarked at a plant (regional center) fifteen miles away.

I'll use the location that I worked at as an example. In Wilmington, NC, once your carrier returns to the station, they drop your letter into a collection bin. This collection bin is driven to Fayetteville, NC, where it is then processed via OCR. After being OCR'd it has a machine-readable bar-code, and will be sorted for delivery to a local station. The machinery involved in the OCR and sorting process is larger than any of the local stations I worked at. Additionally, almost all of the mail pieces that I delivered were intercity, not intracity. Your use case is the exception, not the rule.

There are many exceptions and corner cases to what I described, but that's what the process looks like for most mail pieces. Locating the scanning and sorting machinery at a hub rather than each individual spoke makes sense. UPS, where I currently work, has some minor differences, but the hub and spoke model is largely the same.

At some point in the past, the balance of intracity vs intercity mail would have been different.

Now, utility bills come from some distant office, as do letters from banks, and we don't send invitations very often.

X years ago, bills were sent by closer companies, the bank was local and it was polite to invite someone to dinner by letter.

With that, it would have made sense to separate local and other mail as soon at the postman brought it to the post office.

Both of your observations are due to constant relentless cost optimization within the postal system. Local post offices, for the most part, don't postmark mail at all. Everything gets batched to the regional processing center where it goes through the big automated sorting machines.

I seem to remember a "local only" box in the town where I grew up but that hasn't been a thing for decades now.

A friend lived in a smaller town where mail was processed in the nearest larger city (in a neighboring state). But, if you needed something sent to (most of) the rest of the state, it was faster to have it processed in a different city in the other direction. They had a specific drop box for that, which cut delivery from two days to one. I, living in the second city, was stuck with having my mail routed out of state to get to him.

FWIW, early 2000s where I lived had a local only chute. When I was <10 I always wanted to put the letters in the box.

Knowing that our mail passes through the regional processing center before returning to the local area, we expect about three days if we mail a neighbor.

Post offices around us have two mail slots when you walk in to drop mail in the box: local only, and everywhere else. Presumably they have (once had?) separate localized processing for those in the “local” bin. Because of post office closures and reduced hours, I think everything is just passed off to the regional facility for the last few years.

Another interesting discovery: I live about 3mi from my parents. They’re over the line in another county. My post office services them, but when I still lived in their house, all our local government business (schools, taxes and whatnot) was in the county seat about 15mi further away from the serving post office, closer to the next regional facility further south. If we ever dropped mail, say from mom to her mother, in that town, it took at least an extra day because it had to hit two regionals before getting to grandmother’s local.

Ah, don't you need to know the use case before you condemn the solution? Inefficient yes, but still the best solution for this edge case. The ellipses were a lightly informed nod to the explanations offered by other commenters.

I’m not condemning the solution. I’m condemning the implied critique of a system that you admit worked even in your edge case.

I was not critiquing the USPS. The parent comment observed that the USPS no longer can complete delivery on a partial address. I was suggesting an explanation, i.e., sorting no longer is done by people with the local context necessary to complete such a delivery. That was confirmed by other comments, and indeed suggested by the article.