Shipment.cs

public class UpdateShipmentStatus
{
    public int ShipmentId { get; set; }
    public ShipmentStatus Status { get; set; }
} 

public class ShipmentHandler
{
    private readonly IRepository<Shipment> _shipmentRepository;

    public ShipmentHandler(IRepository<Shipment> shipmentRepository)
    {
        _shipmentRepository = shipmentRepository;
    }

    public void Handle(UpdateShipmentStatus request)
    {
        var shipment = _shipmentRepository.Get(request.ShipmentId);
        shipment.UpdateStatus(request.Status);
        _shipmentRepository.Save(shipment);
    }
}

public enum ShipmentStatus { ArrivedAtShipper, DepartedShipper }

public class Shipment : Entity
{
    public ShipmentStatus CurrentStatus { get; private set; }

    public void UpdateStatus(ShipmentStatus newStatus)
    {
        CurrentStatus = newStatus;
    }
}