public CheckoutTotal CheckOut()
 {
     var preTaxTotal = _checkoutProducts.Sum(p => p.Price);
     _total = new CheckoutTotal(preTaxTotal);
     _total.CalculateTax(_state);
     return _total;
 }
 public CashRegister()
 {
     _checkoutProducts = new List<Product>();
     _total = null;
 }
 public Receipt(IEnumerable<Product> products, CheckoutTotal total)
 {
     _products = products;
     _total = total;
 }