Shipment.cs

public class Shipment
{
    public int ShipmentId { get; private set; }
    public Shipper Shipper { get; private set; }
    public ICollection<Stop> Stops { get; } = new List<Stop>();

    public Shipment(int shipmentId, Shipper shipper)
    {
        ShipmentId = shipmentId;
        Shipper = shipper;
    }

    public bool IsComplete()
    {
        return Stops.All(x => x.Status == StopStatus.Departed);
    }

    public void Arrive(Guid stopId)
    {
        var currentStop = Stops.SingleOrDefault(x => x.StopId == stopId);
        if (currentStop == null)
        {
            throw new InvalidOperationException("Stop does not exist.");
        }

        var previousStopsAreNotDeparted = Stops.Any(x => x.Sequence < currentStop.Sequence && x.Status != StopStatus.Departed);
        if (previousStopsAreNotDeparted)
        {
            throw new InvalidOperationException("Previous stops have not departed.");
        }

        currentStop.Arrive();
    }

    public void Pickup(Guid stopId)
    {
        var currentStop = Stops.SingleOrDefault(x => x.StopId == stopId);
        if (currentStop == null)
        {
            throw new InvalidOperationException("Stop does not exist.");
        }

        if (currentStop.StopType == StopType.Pickup)
        {
            throw new InvalidOperationException("Stop is not a pickup.");
        }

        currentStop.Depart();
    }

    public void Deliver(Guid stopId)
    {
        var currentStop = Stops.SingleOrDefault(x => x.StopId == stopId);
        if (currentStop == null)
        {
            throw new InvalidOperationException("Stop does not exist.");
        }

        if (currentStop.StopType == StopType.Delivery)
        {
            throw new InvalidOperationException("Stop is not a delivery.");
        }

        currentStop.Depart();
    }

    public void UpdateShipper(Shipper shipper)
    {
        Shipper = shipper;
    } 
}