Amazon began delivering packages on Sundays in New York and Los Angeles last year, and today it's expanding Sunday deliveries to 15 more cities, including Philadelphia, Indianapolis, and six locations in Texas. As before, Sunday deliveries won't cost extra — they'll simply be delivered as usual, if that's the day that Amazon and the US Postal Service, its delivery partner, happen to get to them. Apparently baby supplies have been the most common Sunday deliveries so far.

USPS deliveries when you'd least expectThe other cities gaining Sunday delivery include Austin, Dallas, Houston, San Antonio, Waco, and College Station in Texas, as well as Cincinnati and Columbus, Ohio, Lexington and Louisville, Kentucky, New Orleans and Shreveport, Louisiana, and Oklahoma City.

Amazon has been increasingly aggressive about getting ahead of its competitors on delivery times — a move that could ultimately be the difference between it remaining the dominant online retailer or some other company moving in on it. Other companies are already attempting one-hour delivery services in major cities, but Amazon has something faster and more ambitious in mind for down the road: drones that bring packages to your door in half an hour. That's likely still a ways away from reality, but for now, Amazon's Sunday delivery service is another simple but compelling reason to place your orders on its site. At least, if you live in the right location.