Instead, residents in these 14 regions—including New York, Atlanta and Boston as well as new same-day delivery markets Tampa Bay in Florida and San Diego—will now receive free same-day delivery to members on orders of $35 or more. The service previously cost $5.99.  "Basically what we're announcing just makes it easier yet again for Amazon Prime members," said Greg Greeley, vice president, Amazon Prime.  Read MoreWal-Mart's woes: 5 ways it's trying to fix them  Retailers have been upping the ante on delivery deals to gain an edge as more shopping is done online. Earlier this month, Wal-Mart said it is testing a subscription model that would provide free shipping for an annual fee of $50—about half the price of an Amazon Prime membership. And after offering free shipping on all Web orders during the holidays, Target in February lowered its free shipping minimum to $25 from $50.  Amazon, which is also up against traditional retailers' ability to quickly fill customers' orders at their stores, has likewise been sweetening the deal for its shoppers. In December, it launched Prime Now, which provides one-hour delivery on everyday household items including paper towels and batteries for $7.99; two-hour delivery is free. The program, which initially kicked off in Manhattan, has since expanded to cities including Miami and Dallas.  Read MoreWhy you're looking at online sales all wrong  Amazon has also increased its investment in fulfillment centers across the U.S., and now has more than 50 locations. On Tuesday, the company said it would hire more than 6,000 full-time workers across its network to "meet growing customer demand."
Amazon Prime members living in more than a dozen metropolitan areas will no longer have to wait two days to score free shipping.