Instant gratification is a little closer at Amazon.com (Nasdaq: AMZN). The leading online retailer is now offering same-day delivery in select cities -- but it'll cost you.

Baltimore, Boston, Las Vegas, New York City, Philadelphia, Seattle, and Washington, D.C., are the first metropolitan hubs to get the service this week. Eligible orders placed on weekday mornings, with cutoff times between 10 a.m. and 1 p.m., depending on the city, will arrive no later than 8 p.m. that day.

Amazon will have to do a lot of legwork to make it all possible, and customers will of course foot the bill. If you want a new CD, for example, the same-day delivery fee is $10.99 for the shipment plus$1.99 per item.

I don't care how big of a Breaking Benjamin fan you may be: You're probably not willing to pay $12.98 to ship an $11.99 copy of the band's new Dear AgonyCD.

The math gets kinder on bigger-ticket items, but delivery rates also go up by category. Automotive parts, for example, incur a $14.50 shipment fee along with $3.99 for every pound of merchandise ordered.

The new offering is a sweeter deal for Amazon Prime members. Those who pay $79 a year in exchange for free two-day shipping and $3.99 for overnight deliveries of Amazon-stocked goods will pay just $5.99 per item for the service. So this move will probably drum up membership sales.

The timing of the rollout is also important, since Amazon knows it's going to bump into some desperate holiday shoppers come late December.

Conventional retailers aren't going to flinch initially. If someone would rather pay more than $20 to have an Amazon-delivered MP3 player than to head out to the nearest Target (NYSE: TGT) or Best Buy (NYSE: BBY), that customer wasn't going to be an in-store shopper anyway. However, just as the warehouse-club nature of Costco (Nasdaq: COST) or BJ's Wholesale (NYSE: BJ) drives dues-paying members to lean on the clubs for more of their shopping needs, more Amazon Prime signups will mean more market share pain for the bricks-and-mortar retailers. Continued...