As with the rest of the modern world, junk mail (advertisers) are the backbone of the USPS. Cutting down the amount of junk mail might be bad for the post office.
I disagree. The post office depends on junk mail only so long as it has to make door to door deliveries. If the door to door delivery is paid for by the subscriber directly the USPS can ditch junk mail as the main source of income. My main point is that USPS relying on junk mail is not sustainable. Either it will go bankrupt, or it will get huge tax subsidies, or it will get a bit creative. It is going to lose some of its work force either way.