test.cs

public class Order {
  public int Id { get; set; }
  public DateTime OrderDate { get; set; }
  public ContractParty Buyer { get; set; }
  public ContractParty Seller { get; set; }
  public Decimal Allowance { get; set; }
  public Decimal Vat { get; set; } = 6.5M;
  public List<LineItem> LineItems { get; set; } = new List<LineItem>();
  
  public Decimal LineTotalAmount {
    get {
      return this.LineItems.Sum(item => item.GrossPrice * item.Quantity);
    }
  }

  public Decimal TaxTotalAmount {
    get {
      return (this.LineTotalAmount / 100) * this.Vat;
    }
  }

  public Decimal GrandTotalAmount {
    get {
      return this.LineTotalAmount + this.TaxTotalAmount;
    }
  }

  public class LineItem {
    public string Name { get; set; }
    public string Description { get; set; }
    public QuantityCodes QuantityCode { get; set; } = QuantityCodes.H87;
    public int Quantity { get; set; }
    public decimal GrossPrice { get; set; }
  }

  public class Seller : ContractParty { }
  public class Buyer : ContractParty { }

  public class ContractParty {
    public int Id { get; set; }
    public string Name { get; set; }
    public string PostalCode { get; set; }
    public string City { get; set; }
    public string Address { get; set; }
    public CountryCodes CountryCodes { get; set; }
    public ContractPartyContact BuyerContact { get; set; }
  }

  public class ContractPartyContact {
    public string FirstName { get; set; }
    public string LastName { get; set; }
  }
  
}