Read the Postal Reorganization Act of 1971, they are supposed to be self sufficient, and not make a profit or lose money in the long term. The problem is Congress won't let them operate efficiently. When they tried to close the one horse town post offices that cost about $100,000 a year to operate, the Congress stopped them because of public out cry. The people would still get mail service, it would be rural routes, but the want their post offices.
When expense increase, the post office cannot raise prices until they go though the postal rate commission which is controlled by Congress. While Fedex and UPS can change rates as needed, the post office has to wait on Congressional approval.
If the post office is privatized, the expense to the taxpayers will increase. The reason being, no company will take over the remote rural areas that the post office now serves 6 days a week. The only way the remote area will get service will be with government subsidies, that will come from our tax dollars. Just like Fedex and UPS uses the post office today to deliver to remote areas so will the replacement companies.