As of October 2014, the Sunday delivery option is available only in select cities. Senders who choose this option pay the USPS an additional fee. With this option, senders have to select Priority Mail Express as their shipping speed.
Online retailer Amazon announced in 2013 that it would be expanding its delivery options to include Sunday delivery in select cities. This delivery option was the result of a partnership between Amazon and the USPS and was not available nationwide. Amazon does not charge customers an additional fee for Sunday delivery and also offers this option as a part of its Prime shipping service.