A sign is seen outside a Macy's retail store in San Francisco, CaliforniaVia Phil Wahba and Dhanya Skariachan NEW YORK (Reuters) – U.S. shops have little room for error within the quick-coming near and shortened holiday buying season, a period that usually generates 30 percent of annual sales. A late Thanksgiving – the unofficial kickoff for holiday gross sales – has lopped six days off the gift-buying season. The fight for retail greenbacks is so fierce that even Uncle Sam shall be handing over on Sundays in some locations this vacation season. Amazon.com Inc announced on Monday that it could be the first retailer to accomplice with the U.S. Postal Provider to supply Sunday supply starting November 17 in the New York and Los Angeles areas as it seems to be to present customers more transport options.