Added to the list: Austin, Houston, College Station, Dallas, San Antonio and Waco in Texas; Cincinnati and Columbus in Ohio; New Orleans and Shreveport in Louisiana; Lexington and Louisville in Kentucky; Indianapolis, Ind.; Oklahoma City, Okla.; Philadelphia, Pa.
The U.S. Postal Service said some of these cities have been getting Sunday service since February, while others were more recent.
The most common items delivered on Sundays include clothes for newborns, baby books and toys.
"Sunday delivery is clearly crossing errands off the weekend to-do list," said Mike Roth, Amazon's vice president of North America operations.
Sunday delivery comes with the same shipping charges as any other day of the week. Amazon Prime members, who pay an annual fee to qualify for free shipping and other perks, will be able to place orders on Fridays for Sunday delivery.
USPS spokeswoman Sue Brennan called the expanded delivery a "winning proposition" for Amazon.
It's a low-wage recovery
"As online shopping continues to increase, the postal service is happy to offer shipping solutions that allow shippers and customers alike appreciate the benefits of the U.S. mail," Brennan said.
The U.S. Postal Service has been in a financial bind as fewer consumers send letters and bills through first-class mail -- the kind that pays the agency's bills. On top of that, the postal service has had to make multi-billion dollar prepayments toward funding health care benefits for future retirees. In 2013 it lost $5 billion.
At the same time, the postal service saw 8% growth in package delivery in 2013, marking its best year since the beginning of the recession.
Neither Amazon nor the USPS would discuss the terms of their service agreement, nor would they say how many packages are expected to be delivered on Sundays this year.