This is long and you may not want to read it, this isn't a simple "business" decision. There are several things going on:
1) the USPS used to have a surplus and a big pension fund, and any pool of money is going to foment dogfights in DC (at one point the USPS had to pay for the pensions for employees' FORMER jobs in the military to plug a federal budget problem).
2) Congress has Constitutional oversight over the USPS, even though it's an independent agency that doesn't use tax funds (except for those awful mailings from Congressmen they don't have to pay for, and to blind people).
3) Republicans HATE the post office, with its hundreds of thousands of unionized employees and solid path to the middle class, and would love to use it as another laboratory for their privatization and outsourcing experiments
4) Private business--UPS and FedEx--would love a piece of what they know is a profitable business, and give twice as much in campaign donations to Republicans than they do to Democrats.
Remember the 2006 election? Democrats won majorities in both the House and the Senate, so it was the end of that era when Republicans held the Presidency, Senate, and House.
In December 2006 the lame-duck Republican Congress rushed through, and Bush signed, a crippling piece of legislation called the Postal Accountability & Enhancement Act. It requires the USPS to save up (in just 10 years) the next 75 years of employees' pension payments. Which, of course, no other agency has to do, and I doubt any private business does.
It's those payments that make it a money-losing operation currently. If they didn't have that onerous obligation in 2012, there would have been a $1.2 Billion SURPLUS. They've done an admirable job of modernizing and automating and running it as a business.
USPS would like to be even more entrepreneurial while continuing to provide universal service--like postal services in other countries do successfully--but guess what?
The Republicans' 2006 law forbids that; they can't do anything non-postal like offering lines of insurance or savings accounts or logistics consulting. And why can't USPS ship beer and wine? Congressional prohibition.
So, what have Republicans achieved?
-Elimination of up to 200,000 union jobs
-Protection of competition to shippers, banks, insurers and other campaign-contribution industries
-With this no-Saturday thing, a $5 Billion loss of revenue that is not going to help the balance sheet any
-A hastening of the time when Republicans can start suggesting USPS be privatized