Well for one thing, the private nature of the card companies means that they can be 'pressured' by the government and they have no accountability. If the government operated the payment card systems, the constitutional constraints would apply (so for example, wikileaks could have some shelter under the first amendment). As for the postal system, it's being sabotaged from within - http://my.firedoglake.com/mmonk/2011/08/19/the-pre-funding-m...
The legal restitution is likely to be identical, as this is a post-Patriot act age of gag orders. However, with multiple competitors it's a lot harder to shut down something like wikileaks in a completely silent and uniform manner, without indication that it's been silenced.
Most important of all, however, is that a us gov monopoly precludes the use of foreign competitors from countries with better speech laws.
As far as the post office is concerned: Leaving their numerical financial situation aside (which is hard to ascertain exactly due to the opaque nature of government spending) there is simply no excuse for why, after decades and decades, they haven't implemented any of the modern efficiency mechanisms or services that modern post offices have.
Also as far as the post office is concerned: one troubled branch of the government != indictment of all government involvement in public utilities (which is what a government-operated payment card system would amount to). Not every government project is a bureaucratic failure. For example, this internet thing we're using to argue about how the government can't do anything right...
