OOP Design on shopping cart and discounts application

My application needs to have the ability to place different types of discounts with different types of rules to a shopping cart.

I have finished writing the application, but I know there are major flaws in the design I have come up with, from my understanding the Decorator Pattern would solve most of those flaws, but I'm having some trouble on applying it.

Here is in short the code I believe to be flawed

public interface IDiscount
{
    decimal Value { get; }
    Cart Cart { get; set; }

    void ApplyDiscount();
}

public class SimpleDiscount : IDiscount
{

    public void ApplyDiscount ()
    {
        Cart.DiscountAmount += Value;
        Cart.Discounts.Add (this);
    }

}
public class ItemDiscountDiscount : IDiscount
{

    public void ApplyDiscount ()
    {
        if(Cart.HasItem(this.Item)){
            Cart.DiscountAmount += Value;
            Cart.Discounts.Add (this);
        }
    }

}


public class Cart
{   
    internal decimal TotalValue() {
        return this.CartLines.Sum ((cl) => (cl.Price));
    }

    internal decimal FinalValue(){
        return TotalValue - DiscountAmount;
    }

    internal void AddDiscount(IDicount discount){
        discount.Cart = this;
        discount.ApplyDiscount ();
    }

}