And you can help the Postal Service out in keeping prices low even further: Open that bulk mail, grab the "business reply" envelope... AND MAIL IT BACK! Costs you nothing but a few moments of your time, irritates the spammer slightly, and costs them a little money, which goes to support the Postal Service! Win, win, win.
Not if you ask USPS. They aren't supposed to cross-subsidize using services they have a monopoly on (first class and standard mail which includes bulk advertising). Awhile back UPS, FedEx, and others asked the Postal Regulatory Commission to look into whether or not they are doing that. I'm not sure if anything ever came of it.
It doesn't directly subsides it, however it keeps the overall volume up. It costs them about the same to deliver 1 letter to you as it does 5. It also costs them even if they deliver nothing to you. So in that regard, the bulk mail does help out.