Wal-Mart (WMT) might be joining the same-day delivery battle: hungry to gain ground against Amazon (AMZN), the retail giant is testing a same-day service that relies on its stores, costs users $10, and features a 4-hour window. Google, eBay, and many small online retailers are also experimenting with same-day, in spite of the logistics and cost challenges. Amazon remains coy about the extent of its ambitions. "How much time are you saving by not going to the store? People want it now, they want it fast," says an eBay spokeswoman.