Drone swarms are teams of autonomous unmanned aerial vehicles that act as a collective entity to perform a certain task. The objective is to compose Swarm-based Drone-as-a-Service (SDaaS) services for delivery purposes. The composed service will consist of a swarm of drones working together to deliver services from one point to another in a skyway network. We take into consideration the highly constrained environment surrounding the delivery. This includes intrinsic constraints, e.g. different rates of power consumption and window constrained arrival time, and extrinsic constraints, e.g. weather conditions. We will consider several quality of service metrics to select the best SDaaS. The outcome of this research is an efficient infrastructure that shall create new opportunities in smart cities.