[yummyfajitas]

One fairly simple solution would be to reduce the frequency of mail delivery. Instead of Mon-Sat, we could deliver mail Mon-Wed-Fri or Tues-Thurs-Sat (different days for different zones).

[cosmicray]

That doesn't reduce the mail volume. Then you would have to store the mail somewhere, and still have the same amount to sort. So on the days when they did deliver, it would take longer to sort it.

[hexis]

I think one of the premises of the OP was that mail volume will be declining over time due to digital communication.

[stretchwithme]

The commercial mailings could be given to the post office only on the days they'll be delivered. And this commercial mail is often already sorted

Yes, it doesn't reduce mail volume. That's been collapsing all on its own, as has the effort to sort it.