OrderCheckout.cs

public class Order
{
    //properties removed for brevity
    public decimal Checkout()
    {
        decimal amountToPay = 0;

        foreach (var orderLine in this.OrderLines)
        {
            decimal discountAmount = GetApplicableDiscount(orderLine);
            decimal productPriceAfterDiscount = ApplyDiscount(orderLine, discountAmount);

            amountToPay += productPriceAfterDiscount;
        }

        if (Customer.IsPrivilegeCustomer)
            ApplyAdditionalDiscount();

        return  _orderAmount = amountToPay;

        decimal ApplyDiscount(OrderLine orderLine, decimal discountAmount)
        {
            return (orderLine.Price - discountAmount) * orderLine.Quantity;
        }

        decimal GetApplicableDiscount(OrderLine orderLine)
        {
            return (orderLine.Price * orderLine.Discount) / 100;
        }

        void ApplyAdditionalDiscount()
        {
            amountToPay = (amountToPay * 5) / 100;
        }
   }
}