What about other sources of revenue? Establishing an honest notary service, cashing most checks, selling fishing and hunting licenses, wrapping holiday gifts, and accepting wine or beer for delivery are just a few congressionally prohibited proposals that have been put forward by postal activists and watchdogs.
The conservative anti-government strategy is to set government up to fail (usually by starving it of funding). Then they point to the resulting "crisis" they created and say that it proves that government doesn't work and that we should therefore "privatize" it. Now they're coming for the U.S. Postal Service.