ventory and function as a walkie-talkie and cash register. Nordstrom is introducing mobile devices that allow salespeople to check inventory on behalf of a customer and make the sale right then, register required.Starbucks is joining in on the self-service checkout, too, with mobile payments that allow customers to pay from their smart phone. No need for cash, credit cards, or paper receipts. Apple stores already use portable tablets with credit card readers.Self-checkout saves time, reduces errorsThe system works out well for retailers, which have taken a one-two punch from both the popularity of online shopping and the way smart phones enable consumers to comparison shop while in the store, looking at listings from competitors for lower prices before deciding to buy. In the sixth annual survey from National Retail Federation Foundation and American Express Customers' Choice survey, conducted by BIGresearch, Zappos.com and Amazon.com took the top two spots.Shoplifting hasn't been an issue, either, at least not for Ahold, which spot checks receipts. According to research from information technology research firm IDC, retailers lose more money from cashiers ringing up prices incorrectly than from shoplifters using mobile checkout technology.Convenience can lead to overspendingConsumers looking for ways to stash more cash in their savings accounts, however, should know they might wind up buying more than they intended while using the devices. With the Scan It system, says Erik Keptner, Ahold's senior vice president for marketing and consumer insights, consumers spend an average of 10 percent more than the average shopper. Keptner attributes this to the feeling of control the scanner gives consumers and the coupons that pop up as they make purchases for related items.However, one shopper interviewed said she has reduced both the time it takes to shop and her total, saving 20 minutes and 5 percent.Eventually the handheld gadgets will likely be replaced by smart phone