public async Task TestShouldGetBill()
        {
            List <Item> items = new List <Item>();

            items.Add(new Item()
            {
                Price = 30.0, Quantity = 9, Description = "product-a"
            });
            items.Add(new Item()
            {
                Price = 14.0, Quantity = 16, Description = "product-b"
            });
            items.Add(new Item()
            {
                Price = 3.90, Quantity = 42, Description = "product-c"
            });
            items.Add(new Item()
            {
                Price = 6.99, Quantity = 12, Description = "product-d"
            });

            // create a bill then retrieve it through the get method - they should match
            var bill = new Bill()
            {
                Number   = "6",
                Currency = Currency.USD,
                Email    = "", //email address mandatory
                Items    = items
            };
            var basicBill = await _bitpay.CreateBill(bill);

            var retrievedBill = await _bitpay.GetBill(basicBill.Id);

            Assert.AreEqual(basicBill.Id, retrievedBill.Id);
        }