LOL, as a stamp collector, I try to use stamps on my mail. Often old stamps as well as new ones. Even if we are generous and say there are 100,000 stamp collectors in the US, that is still a tiny fraction of the population!
You've opened a Pandora's box of issues. Let me list some of them.
Unfortunately, you are right in that most people rarely send letters, but only bills, by mail. Until the Lovejoy changes, that is. After that, a lot of people began paying bills online if they could. I have not done online transactions for more than a year, after some of my accounts got hacked, so I personally deliver the bills I can pay in town, and use telephone banking for the one monthly out-of-state payment I need to make. But people do send parcels by mail and receive them from businesses and online vendors -- so the USPS should make money on them although they don't have to stick physical stamps on them. In fact, I believe a lot of their income is from parcel post rather than letters. They also have competition from UPS, FedEx, etc.
Another reason why the USPS has been in deficit for a long time is because Congress ordered them to fully fund the retirement systems of all postal employees well into the future, with zero flexibility. This led to a lot of their revenues from their operations being sunk into the retirement system, resulting in mounting current deficits.
There is another problem as well, since you bring up politics. As a capitalist society, many among us are inclined to think of every human endeavour as a profit-making business enterprise, and the outcomes of these ventures as products. As a former academic, I have had numerous discussions with business people who think that universities are factories producing educated students. Sometimes they say that the students are consumers, and their satisfaction is the goal of all education, so students' demands must be satisfied.
In a similar vein, many legislators and politicians have come to regard the USPS as a business which should make a profit. It is not -- it is a service, in the same way that the US Military is a service to the nation. Some things simply are not businesses. There is talk of privatizing the postal service. Would we privatize the military? Some private armies like Blackwater were used by the US in operations in Iraq and that has led to all sorts of problems. We have already privatized the prison system and look at the horrors that has led us to, with the conditions in detention centers and private prisons!
So there are many sides to the postal delay story.