public void ShoppingCart_buy_1_book()
        {
            //arrange
            ShoppingCart target = new ShoppingCart();
            PotterBook   book   = new PotterBook {
                Episode = 1
            };

            target.AddToCart(book);
            var expected = 100;

            //act
            var actual = target.Billing();

            //assert
            Assert.AreEqual(expected, actual);
        }
        public void ShoppingCart_buy_2_different_books()
        {
            //arrange
            ShoppingCart      target = new ShoppingCart();
            List <PotterBook> books  = new List <PotterBook>
            {
                new PotterBook {
                    Episode = 1
                },
                new PotterBook {
                    Episode = 2
                },
            };

            target.AddToCart(books);
            var expected = 190;

            //act
            var actual = target.Billing();

            //assert
            Assert.AreEqual(expected, actual);
        }