You model a dial-a-ride problem much like a vehicle routing problem with pickups and deliveries. The capacity of vehicles can be interpreted as number of seats available. A shipment is here understood as a ride from one location to another (probably you want the shipment to have a capacity-demand of 1). See [VRP with pickups and deliveries](https://github.com/jsprit/jsprit/wiki/VRP-with-pickups-and-deliveries).