Order Model

Order.cs

public class Order
{
  public string Id { get; private set; }
  public string Number { get; private set; }
  public string CustomerId { get; private set; }
  public DateTime Date { get; private set; }
  public ICollection<OrderLineItem> Items { get; private set; }
  public decimal ShippingCharge { get; private set; }
  public decimal TaxCharge { get; private set; }
  public decimal Total
  {
    get { return this.Items.Sum(x => x.TotalPrice) + this.ShippingCharge + this.TaxCharge; }
  }
}

public class OrderLineItem
{
  public Product Product { get; private set; }
  public decimal Price { get; private set; }
  public int Quantity { get; private set; }
  public decimal TotalPrice
  {
    get { return this.Price * this.Quantity; }
  } 
}

public class Product
{
  public string Id { get; private set; }
  public string Brand { get; private set; }
  public string ModelNumber { get; private set; }
}