Amazon began delivering packages on Sundays in New York and Los Angeles last year, and today its expanding Sunday deliveries to 15 more cities, including Philadelphia, Indianapolis, and six locations in Texas. As before, Sunday deliveries wont cost extra — theyll simply be delivered as usual, if thats 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. 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.