NotFoundCustomer.cs

public interface Customer
{
    double GetEarnedDiscount();

    void AddToOrdersHistory(Order order);
}

// Null Customer's implementation
public class NotFoundCustomer : Customer
{
    private readonly double DEFAULT_DISCOUNT = 1.0;

    public double GetEarnedDiscount()
    {
        return DEFAULT_DISCOUNT;
    }

    // Refused bequest???
    public void AddToOrdersHistory(Order order)
    {
       
    }
}