public void CalculateTotalPriceTest()
        {
            //Arrange.
            string         input          = "[Apple, Apple, Avocado, Apple, Apple]";
            BillingService billingService = new BillingService(input);

            //Act.
            var totalPrice = billingService.CalculateTotalPrice();

            //Assert.
            Assert.AreEqual(totalPrice, 0.85m);
        }