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.
