That service is an upgrade of the company's subscription "Amazon Prime" offering, which offers unlimited, free two-day shipping on a broad list of items. The "Amazon Flex" app allows users to register their names, drive to a distribution center, scan outgoing packages with their camera phones, receive turn-by-turn driving directions to buyers' homes, and receive payment when they have delivered each parcel. Drivers must submit their driver licenses and other data to undergo a 48-hour background check before receiving their first assignments, Clark said, keynoting the second day of the annual conference of the Council of Supply Chain Management Professionals (CSCMP) in San Diego. According to Clark, "Amazon Flex" is an inexpensive way to support the "Prime Now" offering, allowing the company to manage the variability of customer demand by hiring temporary delivery agents instead of building a permanent infrastructure that could suffer from underutilized capacity during slow times.