The aim of this study is to explore the potential of using public transportation systems for freight delivery, where we intend to use the spare capacities of the public transportation vehicles like buses, trams, trains, etc., particularly during off-peak hours, to transport packages within the city, instead of dedicated delivery vehicles. The study contributes towards the growing body of literature on innovative strategies for performing sustainable last-mile deliveries. We study an operational level problem, called the Three-Tier Delivery Problem on Public Transportation, where packages are first transported from the Consolidation and Distribution Center (CDC) to nearby public vehicle stations by the delivery vehicles, and from where they are transported into the city by the public vehicles. Then the last leg of the delivery is performed to deliver the packages to their respective customers using green vehicles or eco-friendly systems. We propose mixed integer linear programming formulations to study the transport of packages from the CDC to the customers, use decomposition approaches to solve them, and provide numerical experiments to demonstrate the efficiency and effectiveness of the system. Our results show that this system has the potential to drastically reduce the length of trips performed by dedicated delivery vehicles, thereby reducing the negative social and environmental issues caused by existing last-mile delivery systems. We propose mixed integer linear programming formulations to study the transport of packages from the CDC to the customers, use decomposition approaches to solve them, and provide numerical experiments to demonstrate the efficiency and effectiveness of the system. Our results show that this system has the potential to drastically reduce the length of trips performed by dedicated delivery vehicles, thereby reducing the negative social and environmental issues caused by existing last-mile delivery systems. We propose mixed integer linear programming formulations to study the transport of packages from the CDC to the customers, use decomposition approaches to solve them, and provide numerical experiments to demonstrate the efficiency and effectiveness of the system. Our results show that this system has the potential to drastically reduce the length of trips performed by dedicated delivery vehicles, thereby reducing the negative social and environmental issues caused by existing last-mile delivery systems.