So the postal service loses money because it's mandated by law that it has to serve the entire country. But mandating that it has to provide banking services for the entire country is going to save money? Ok.
Providing basic banking services at a post office is not nearly as big a proposition as delivering mail to every address in the country.
Plus banking services are touched less frequently than mail. I get junk mail every day. I don't know the exact logistics of what they're suggesting, but I have a hard time seeing how it could be as expensive as mail delivery.