Exclusive: Amazon expanding deliveries by its 'on-demand' drivers

By Mari Saito SAN FRANCISCO (Reuters) - Amazon.com Inc is quietly inviting drivers for its new "on-demand" delivery service to handle its standard packages, as the online retailer known for low prices and razor-thin profit margins looks to speed up delivery times and tamp down its growing multi-billion dollar logistics bill. Amazon outlined details of its latest plan over the last few weeks in an email to contract drivers who deliver parcels for Amazon Flex, a program launched last year to handl