I recently signed up for Amazon.com Prime, their new flat-rate shipping service. For $79 a year, all of my orders ship with 2nd day delivery for free. Overnight costs $3.99. I spent far more than that on shipping in the last 12 months, and usually use ground, so it made plenty of financial sense.
I've placed a few orders, and so far everything has gotten here on time. What I just noticed today, though, is that my most recent order actually shipped out UPS Ground. It just happened to ship from a warehouse that was 2 days away via ground service.
As long as it arrives in two days, I don't care how they get it here. What I find interesting though, is that if they have the capability to calculate shipping times for various products from various warehouses to various destinations (customers) why don't they offer optimized shipping? Rather than pay to upgrade for overnight shipping, if a warehouse is overnight via ground service for a customer, let them know ahead of time. They could then offer cheaper ground service that is still overnight for many people.
Of course, this would kill some of their markup and that from UPS, so it is understandable why they wouldn't want to offer such a service.