Multi-modal Logistics Transit Optimizer

Design a system for a global logistics company to manage 'Shipments' that move through various 'Legs' (Air, Sea, Rail, Road). Each leg has different constraints (e.g., refrigerated containers for sea, hazardous material restrictions for air). A shipment might require special 'Add-ons' like insurance, white-glove delivery, or temperature tracking. Design the system to calculate the total cost and validate the feasibility of a multi-modal route.