private static decimal GetTotalPrice(ProductInBasket productInBasket)
 {
     return(productInBasket.Price * productInBasket.Quantity);
 }
        internal static decimal GetTotalDiscount(ProductInBasket productInBasket)
        {
            List <decimal> discountedItems = productInBasket.Discounts.Select(discount => discount.GetDiscount(productInBasket)).ToList();

            return(discountedItems.Sum());
        }