I know it's the norm here to avoid calling out one political party in particular, but it's way past time to point out that it's overwhelmingly Republicans who are crafting regulations to ensure the post office fails and turns from a world-class institution to something that can be pointed to as an example of government failing.
The post master general under Trump was trying to cut costs within the constraints placed on the USPS by congress, which is to reduce mail drop boxes and other activities. If congress wanted to invest in the USPS, they could, it just hasn't been done yet (no one seemed to agree on what that should look like).
One important part with the drop in mail volume is that the cost of mail delivery actually goes up. There are many fixed costs on delivering a letter, and if you can deliver 8 letters vs 4 letters to the same address and are forced to charge the same regardless of how many you end up delivering there, the overall delivery-cost of each letter goes up.
So either we need to let the USPS charge more for their services (to offset the reduced volume), or congress needs to decide to subsidize the USPS again to allow it to function properly.
Subsidizing would be difficult given how UPS and Fedex abuse the USPS today. (Those two carriers will push delivery to the USPS when they know its unprofitable for them to deliver any given package). So any subsidy that it provided to the USPS will also subsidize Fedex and UPS unless the rules are changed there as well.