Shipment.cs

public class Shipment
{
    private readonly IList<Stop> _stops;

    public Shipment(IList<Stop> stops)
    {
        _stops = stops;
    }

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

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

        currentStop.Depart();
    }

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

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

        currentStop.Depart();
    }

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