Amazon (AMZN) awoke this morning to a lump of coal in its stocking as Walmart (WMT) announced the test launch of same-day delivery in several cities. The new service comes just in time for the holidays and highlights the key advantage enjoyed by the world's biggest retailer in the race to get customers what they want, when they want it: location, location, location.

The same-day option, branded Walmart To Go, became available to shoppers in Northern Virginia a week ago and Philadelphia customers two days later. The service is set to launch in Minneapolis today, and in San Francisco/San Jose later this month or early November. For a flat $10 charge, customers can go online and order as many same-day eligible items as they want with no minimum purchase. Categories include electronics, toys, sporting goods, and housewares. If they get their orders in before noon, they can choose a four-hour window for UPS to deliver the order that same day. Walmart says the prices will be the same as in local stores, which will supply the inventory to fulfill same-day orders.

Walmart customers are eager to have the choice of same-day delivery, the company says it found in a survey. This is not surprising, even though most U.S. residents live within driving distance of at least one of the country's nearly 3,100 Walmart Supercenters. Same-day delivery combines the convenience of online ordering and home delivery with something approximating the instant gratification of going to the store. Amazon has tested this option and seems to be working on increasing its delivery speeds by building new distribution centers in California and elsewhere closer to where people live.

Industry analysts (and common sense) say that proximity of goods to customers is essential to shortening delivery waits. But Amazon seems unlikely to catch up to Walmart in its capacity to get its wares close to where people live. Walmart same-day delivery blurs the line between online and offline shopping not just for consumers, but also on the backend by having its stores do double-duty as e-commerce warehouses. Turns out one of the most promising ways to succeed at e-commerce might be to have a real store after all.