[btilly]

If it's cheaper to get it delivered via next day air, then it must have used less energy.

Big if. No shipper ever reveals their true costs to you, they always build in some margin. An amusing illustration of this is UPS. Internally 2-day shipping costs more than overnight (they have to pay for warehouse space to keep the packages around for a day), but they charge more for 1-day.

In the case of Diapers.com, no option is provided to let you get any other kind of shipment speed. That's because they see their competition as regular retail stores. And the cost savings of eliminating the retailer are enough that they can justify much higher shipping costs.

For a realistic comparison see http://en.wikipedia.org/wiki/Fuel_efficiency_in_transportati... (and keep in mind that the true costs for transporting stuff by aircraft can depend more on volume than weight).