While technically it doesn't receive tax money because it must earn a profit, it of course is partly financed by the government. Just one example: I lived in an apartment that was owned by the Swiss Post (close to the post office but a separate building ). With a AAA credit rating, it is of course cheap to buy buildings and make a profit. A regular private company couldn't do this. Well, I benefited as well as the rent was relatively cheap compared to similar apartments.

They are supported by being a monopoly by law on letter correspondence. No need for extra subventions when you can decide yourself on your pricing and quality of service.