Here's how it works: if an item in stock is ordered before 10 a.m. or 1 p.m. (the cutoff time varies by city), Amazon will have it delivered to you that same day.
The same-day delivery service will be available in seven cities, including New York, Philadelphia, Boston, Baltimore, Las Vegas, Seattle and Washington D.C. In the next few months, it will be extended to Chicago, Indianapolis and Phoenix.
The "Local Express Delivery" service is available to Amazon Prime members for $5.99 per item, and the cost is about double that for non-prime members. The New York Times notes,
But non-Prime members will have to be pretty desperate for those Pampers Baby-Dry size 3 diapers: same day delivery will cost $15.
Read more about the new service at Amazon.com.