Amazon has announced a partnership with the US Postal Service that will see the retailer deliver packages on Sundays from now on. The program is rolling out first to customers in the New York and Los Angeles metropolitan areas, but the company plans to extend it to cities including Dallas, Houston, New Orleans, and Phoenix in 2014.

First major push into Sunday deliveries for USPS

"As online shopping continues to increase, the Postal Service is very happy to offer shippers like Amazon the option of having packages delivered on Sunday," said USPS CEO Patrick R. Donahue in a press release; it's unclear whether the service will make similar arrangements with other clients. The Wall Street Journal reports that the deal sees Amazon making use of a "little-known offering available to any shipper," but that this is the first major push into Sunday deliveries for the USPS.

Amazon has been getting more aggressive in its shipping strategy, reportedly spending nearly $14 billion in a three-year warehouse expansion and raising its free delivery threshold from $25 to $35. The Sunday deliveries program could take weekend business away from brick-and-mortar retailers, which hold an advantage on weekends since customers ordering on Fridays typically have to wait until Monday.

Update: Although Amazon's press release led us to believe that Sunday deliveries would only be available to Prime subscribers, a company representative has let us know that this is in fact not the case — all customers will be able to take advantage. We've updated this story to clarify.