Shipment.cs

public class Shipment
{
    private DateTime? _delivery;
    private string _proofOfDelivery;

    public Shipment(Guid shipmentID)
    {
        // Load current state
    }

    public void Deliver(DateTime delivery, string proofOfDelivery)
    {
        if (_delivery != null)
        {
            throw new InvalidOperationException("Already delivered.");
        }
        
        _delivery = delivery;
        _proofOfDelivery = proofOfDelivery;
    }

    public bool IsDelievered()
    {
        return _delivery.HasValue;
    }
}