It's already "nationalized", it's just run as a private corporation rather than a Federal agency. It's actually much better that way; most Federal agencies are horribly mismangaged and wasteful; the USPS is actually extremely efficient and well-run. If it weren't for Congress meddling with it, at the behest of lobbyists, they wouldn't have this problem, and they'd be profitable. Also, the USPS has been independent since 1971; that's long before UPS and FedEx were the heavyweights they are now.
Turning it into a Federal agency wouldn't change Congressional meddling. Congress can just as easily meddle with a Federal agency as with a government-owned corporation, and actually moreso. As a separate entity, it's easy to see how the USPS is doing and it has more isolation from stupid politics; Congress has to actually pass laws and such to affect the USPS's operations and behavior. A Federal agency, OTOH, is completely up to the whims of the guy in the White House (as well as the budget-makers in Congress), and things there can change radically every time someone new is elected or Congress decides to do something stupid like cut their budget. The way it is now, Congress has no real say over the USPS's budget or how they handle their money, except for legal mandates like this stupid pension-funding law. Congress can't just yank their funding for no reason, the way they can with every other Federal agency; the USPS is entirely self-funded, and uses no taxpayer money to operate. Change that to a Federal agency, and its revenues would go into the Treasury, and its operating costs would come out of the Treasury, being entirely comingled. It'd be very easy for Congress to simply defund the USPS (regardless of how much money they're making in revenue), cripple it, then point to that and say "look! It doesn't work! We need to eliminate it!" and then pass a new law to eliminate the USPS altogether, or sell it off to a private corporation.
Maybe you should try actually educating yourself about the USPS and the issues involved, and also about how the US government works (which obviously you don't know much about, since you're not American, obvious by your spelling of "nationalise"), before spouting a bunch of nonsense.
The only way to fix the issues facing the USPS is to fix the US government itself, and the corruption which has completely taken it over. The problems with the USPS are just minor symptoms of much, much larger problems with the US federal government, all caused by extreme corruption, turning into an entirely undemocratic, mercantilist/corporatist (some might say fascist) government. The way I see it, it's entirely hopeless at this point, and the only thing to do is wait for it to collapse under its own weight, just like the Roman Empire did.