Because they are both branches of the government. The USPS has no incentive to tell taxpayers the NSA is spying on them. At least with a private company there is a profit motive, and they could lose customers over bad press.
Furthermore, a private company can just bill the government.
> In December 2012, for instance, Microsoft emailed DITU a PDF invoice for $145,100, broken down to $100 per request for information, the documents appear to show. In August 2013, Microsoft allegedly emailed a similar invoice, this time for $352,200, at a rate of $200 per request. The latest invoice provided, from November 2013, is for $281,000.
And yet, RSA didn't come totally clean. Other companies can be served with gag orders preventing them from informing their customers of what's going on behind the scenes.
Furthermore, the NSA reading email is effectively a data breach, and many companies take the view that hiding a data breach is a good thing - no press about data breach means no bad press means no lost customers.
The USPS is answerable to Congress, and that makes it rather more accountable to the public than a firm that's only answerable to its shareholders (who, in the US at least, have virtually no input into the governance of a firm).
