InvoiceNormal.cs

public class InvoiceNormal {
  private bool isCalculateSpecialTax;

  private List<InvoiceItem> items;

  public class InvoiceItem {
    public int quantity;
    public string description;
    public double pricePerUnit;
  }

  public InvoiceNormal(bool isCalculateSpecialTax) {
    this.isCalculateSpecialTax = isCalculateSpecialTax;
  }

  public double calculateSumPrice () {
    var price = 0.0;
    price = this.calculateSumPriceForNormalCase();
    if (this.isCalculateSpecialTax) {
      price = this.plusVat(price) * 1.05;
    }
    return price;
  }

  private double calculateSumPriceForNormalCase () {
    return items.sum(d => d.quantity * d.pricePerUnit);
  }

  private double plusVat (double price) {
    return price * 1.07;
  }
}