public void AmountToDisplay_Test()
        {
            // arrange
            InvoiceModel model          = new InvoiceModel();
            decimal      amount         = 2.50M;
            string       expectedResult = @"$2.50";

            // act
            string actualResult = model.GetAmountToDisplay(amount);

            // assert
            Assert.AreEqual(expectedResult, actualResult);
        }