As a separate sidebar regarding logistics, it's interesting to note that Wal-mart's shipping component is effectively being subsidized by the federal government, by way of the U.S. Interstate system.

While I'm not so much of a libertarian that I think the Interstate system was a bad idea, it is important to note that the Interstate system created an entire category of business (shipping via truck) that directly harmed two existing industries (shipping via boat, shipping via train) and stunted the growth of a third (shipping via plane). This would be all fine and dandy if shipping via truck were more efficient after considering all externalities. But firstly you have the environmental cost of burning gasoline/diesel, including a not-insubstantial impact on the global climate. And secondly you have the more direct economic cost of road wear.

Road wear is a funny thing. The rule of thumb is that road damage accumulates with the fourth power of the weight per axle. A single car with passengers has perhaps 2,000 pounds spread evenly over two axles, for a road wear of O(10^12) times a tiny constant per mile driven. A large truck, of the kind used by Wal-mart, has perhaps 50,000 pounds spread evenly over eight axles (18 wheels, minus two for the cab weight, divided by two to convert to axles). That's a road wear of O(10^15) times constant per mile driven, or 1,000 times greater than a passenger car.

On rural interstates, trucks form between 10% and 50% of traffic.

Thus almost all highway repair dollars are artificially propping up the trucking industry, creating phony profits for the trucking companies by siphoning tax dollars from citizens.