Mail delivery and shipping are crucial, not the USPS. There are plenty of other private companies that can and do show the ability to do what the USPS does - only more efficiently (and without consistently losing money). At this point in history, there's no need for us to continue supporting the tax-subsidized[1] money pit that is the USPS.
Yet another piece of evidence that private industry is more effective than government-funded programs.
[1] Make no mistake - the USPS might be able to claim they don't receive tax money for operations (technically), but the laws and other perks protecting their monopoly status have a real-world cost, and the net effect is no different from that of a tax.
edit: Ok, it looks like many people disagree. If that's the case, explain yourself instead of simply hitting the downvote button.