public async Task PostSaleInvoice(Sale sale)
        {
            string fullUrl = string.Format(ApiEndPoint, "Sales");
            string data = JsonConvert.SerializeObject(sale);

            using (var http = new HttpClient())
            {
                HttpContent content = new StringContent(data);
                content.Headers.ContentType.MediaType = "application/json";
                HttpResponseMessage response = await http.PutAsync(fullUrl, content);
            }
        }
        /// <summary>
        ///     Populates the page with content passed during navigation.  Any saved state is also
        ///     provided when recreating a page from a prior session.
        /// </summary>
        /// <param name="sender">
        ///     The source of the event; typically <see cref="NavigationHelper" />
        /// </param>
        /// <param name="e">
        ///     Event data that provides both the navigation parameter passed to
        ///     <see cref="Frame.Navigate(Type, Object)" /> when this page was initially requested and
        ///     a dictionary of state preserved by this page during an earlier
        ///     session.  The state will be null the first time a page is visited.
        /// </param>
        private async void navigationHelper_LoadState(object sender, LoadStateEventArgs e)
        {
            try
            {
                // TODO: Create an appropriate data model for your problem domain to replace the Customer data
                CustomerDataItem customer = await CustomerDataSource.GetItemAsync((String) e.NavigationParameter);

                IEnumerable<SaleableItemDataGroup> saleableItemDataGroups =
                    await SaleableItemDataSource.GetGroupsAsync();
                DefaultViewModel["ItemsToSell"] = saleableItemDataGroups;

                var currentSale = new Sale();
                currentSale.Customer = customer;
                DefaultViewModel["Sale"] = currentSale;
            }
            catch (Exception ex)
            {
                Debug.WriteLine(ex);
            }
        }