The rise of online retailers has led to a burgeoning demand for rapid package delivery. Amazon, Google, and other companies are leveraging technological advances to pursue the use of small unmanned aerial systems (sUAS) for delivery of small and lightweight packages. sUAS delivery is appealing to these retailers because of the potential for eliminating the reliance on traditional delivery services and for creating a more efficient, flexible, rapid, and vertically-integrated order-to-delivery process. This paper examines the use of sUAS for on-demand package delivery by evaluating potential concepts of operations. A modeling capability is presented that allows for analysis of the economic, regulatory, and logistical aspects of sUAS delivery systems. A case study of on-demand package delivery in the San Francisco area is presented, including a comparison of sUAS delivery with traditional truck delivery. Metrics including cost per package and average package delivery time are simulated to compare different delivery scenarios.