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.

The quality of service appears to be pretty high, among the best in the world. I don't know about pricing, things in Switzerland are always pretty pricey even compared to the UK

Rampant privatization is not any more enticing than what you describe.

Because there are only two ways of doing things in the world?

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.