Amazon's new $15 fee for same-day delivery not worth it

As a cheapskate, I'm willing to look around for free shipping when I buy stuff online, and I'm willing to wait a week to get my purchase. That's partly why I'm a regular shopper at Amazon.com, which offers free shipping on orders of $25 or more.

But now I learn from the New York Times Bits blog that Amazon is rolling out a service offering same-day delivery in seven cities for $6 per item for Amazon Prime members, who pay $79 a year for free next-day shipping. But non-Prime members will pay $15 for same-day delivery.

I don't know what the average amount is that Amazon customers spend, but spending $15 on a $50 purchase to get it the same day is like a 30% tax. Is it worth 30% more, or whatever it is depending on how much you spend, to get something the same day?