By Mari Saito SAN FRANCISCO - 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 handle speedy deliveries of common household goods to customers using Prime Now, a mobile app that comes with Amazon's popular $99-a-year Prime membership. If the gambit works, industry analysts said it could help Amazon contain its shipping costs, which grew more than 18 percent to $11.5 billion last year.