Not saying it’s a good idea (but tough to implement), the fastest way to cut Postal Service costs is to move to 3 day a week deliveries - and for parcel/off-day deliveries the sender pays more (much like Amazon pretty much pays for the Sunday deliveries by the Post Office.
One carrier could then cover 2 routes, delivering one Monday/Wednesday/Friday and the other Tuesday/Thursday/Saturday. Then you keep a few carriers to do off-day deliveries, but you’re seeing a 25-40% workforce reduction, along with the cost associated with it, the vehicles in the fleet, etc.
I can say from personal experience, the volume of mail that I get is virtually non-existent, other than a few bills that still send by mail, and of course all the “bulk rate” stuff - I have a cluster mailbox and am lucky if I check it more than once a week, so reducing the days they have to deliver wouldn’t bother me one bit.