I will admit that the US postal service is remarkably cheap, though if you find out just how cheap it was twenty years ago you might be shocked. (Of course, this has to do more with the screwed up way funds going to Federal Agencies are handled than the USPS' fault). What is interesting is their unusual delivery abilities.
The post office is better at delivery than they are believed to be.
There are a number of occasions where mail addressed to "The Little Old Lady atop Greentree Hill in Pittsburgh" has arrived. Yet, by the same token, I have had them lose mail on me -- a bill and three issues of Dr. Dobbs' Journal have mysteriously vanished en route to me.
But, compared to the cost, what is even better about the US postal service is that they will ship just about anything. Want to send someone a shoe? Put on an address and more postage than you need for the weight. It will likely arrive. You can do this with most things -- label it, put too much postage on it and it will get there. You can't always count on it being as intact, but for a prank, who would suspect the post office as a partner in crime? I first learned of this trick from the book Kids, Shenanigans.
If you're curious about the limits of the post office's tolerance, take a look at http://www.improbable.com/airchives/paperair/volume6/v6i4/postal-6-4.html from our friends at HotAIR. The most interesting result in the article was the brick, which was pulverized in transit by the DEA.