The answer: very very little.
The US Mail (sans packages, which will continue Saturday delivery) has come down to being a bill collector and the deliverer of advertising. The first is equivalent to the proverbial “bearer of bad tidings,” and the second is a largely irrelevant waste of paper. So why strain the postal workers’ backs, and the postal service’s pocketbook, any more than we have to?
No reason at all.
Except, perhaps, that with the end of weekend delivery, mail will pile up even more for Monday.