(Undated) -- The U.S. Postal Service is starting Sunday delivery of packages for Amazon in the Los Angeles and New York City areas.

The fact there is no extra charge for Sunday delivery under Amazon's deal with the Postal Service could be a real advantage for the online retailing giant this holiday season. UPS doesn't have Sunday delivery and FedEx offers limited Sunday delivery service.

The deal also could give a significant boost to the financially troubled Postal Service. Amazon's Sunday package delivery will expand in 2014 to cities like Dallas, New Orleans and Phoenix.