The focus on faster shipping is one way retailers are catering to shoppers who've become increasingly finicky and impatient. Since the recession, it's not enough to get lower prices; they want the deepest discounts. Quick delivery is important for retailers as they head into the winter holiday shopping season, a time when they can make up to 40 percent of their annual sales. Retailers can't afford a repeat last year when UPS and FedEx failed to deliver some packages by Christmas due to a combination of poor weather and overloaded systems, causing angry customers. Amazon expanded its Sunday delivery service, adding more than 10 distribution centers and 15 smaller sorting centers that sort packages by ZIP code and transport them to U.S.