PayPal REST API calls to get access token and process payment

PayPalRESTAPI.cs

using System;
using System.Text;
using System.Net.Http;
using Newtonsoft.Json;
using System.Collections.Generic;
using System.Web.Script.Serialization;

namespace ConsoleApp14
{
    class Program
    {
        static async System.Threading.Tasks.Task Main(string[] args)
        {
            string PayPalClientId = "xxxx";
            string PayPalClientSecret = "xxxx";

            var payPalTokenModel = new PayPalTokenModel();
            using (var client = new HttpClient())
            {
                // -h
                client.DefaultRequestHeaders.Add("Accept", "application/json");
                client.DefaultRequestHeaders.Add("Accept-Language", "en_US");

                // -u
                var byteArray = Encoding.UTF8.GetBytes(PayPalClientId + ":" + PayPalClientSecret);
                client.DefaultRequestHeaders.Authorization = new System.Net.Http.Headers.AuthenticationHeaderValue("Basic", Convert.ToBase64String(byteArray));

                // -d
                var requestParams = new List<KeyValuePair<string, string>>
                {
                    new KeyValuePair<string, string>("grant_type", "client_credentials")
                };
                var content = new FormUrlEncodedContent(requestParams);

                // post and response
                var webresponse = await client.PostAsync(new Uri("https://api.sandbox.paypal.com/v1/oauth2/token"), content);
                var jsonString = await webresponse.Content.ReadAsStringAsync();

                // deserialize
                payPalTokenModel = JsonConvert.DeserializeObject<PayPalTokenModel>(jsonString);

            }

            if (payPalTokenModel.access_token != null)
            {
                var listItems = new ItemList() { items = new List<Item>() };
                listItems.items.Add(new Item()
                {
                    name = "product_name_here",
                    currency = "INR",
                    price = "100.00",
                    quantity = "1"
                });
                var paymentDetails = new Details()
                {
                    tax = "0",
                    shipping = "0",
                    subtotal = "100.00"
                };
                var amount = new Amount()
                {
                    currency = "INR",
                    total = "100.00",
                    details = paymentDetails
                };
                var transactionList = new List<Transaction>();
                transactionList.Add(new Transaction()
                {
                    invoice_number = "INOVICE_" + Guid.NewGuid().ToString(),
                    amount = amount,
                    item_list = listItems
                });

                var paypalPaymentRequest = new PayPalPaymentRequestModel();
                paypalPaymentRequest.intent = "sale";
                paypalPaymentRequest.payer = new Payer { payment_method = "paypal" };
                paypalPaymentRequest.transactions = transactionList;
                paypalPaymentRequest.note_to_payer = "note here";
                paypalPaymentRequest.redirect_urls = new RedirectUrls { return_url = "https://example.com/returnurl", cancel_url = "https://example.com/cancelurl" };

                using (var client = new HttpClient())
                {
                    // -h
                    client.DefaultRequestHeaders.Add("Accept", "application/json");
                    client.DefaultRequestHeaders.Add("Authorization", "Bearer " + payPalTokenModel.access_token);

                    // -d
                    var requestParams = new JavaScriptSerializer().Serialize(paypalPaymentRequest);
                    var content = new StringContent(requestParams, Encoding.UTF8, "application/json");

                    // post and response
                    var webresponse = await client.PostAsync(new Uri("https://api.sandbox.paypal.com/v1/payments/payment"), content);
                    var jsonString = await webresponse.Content.ReadAsStringAsync();
                }
            }
        }
    }

    public class PayPalTokenModel
    {
        public string scope { get; set; }
        public string nonce { get; set; }
        public string access_token { get; set; }
        public string token_type { get; set; }
        public string app_id { get; set; }
        public int expires_in { get; set; }
    }

    public class PayPalPaymentRequestModel
    {
        public string intent { get; set; }
        public Payer payer { get; set; }
        public List<Transaction> transactions { get; set; }
        public string note_to_payer { get; set; }
        public RedirectUrls redirect_urls { get; set; }
    }

    public class Payer
    {
        public string payment_method { get; set; }
    }

    public class Details
    {
        public string subtotal { get; set; }
        public string tax { get; set; }
        public string shipping { get; set; }
        public string handling_fee { get; set; }
        public string shipping_discount { get; set; }
        public string insurance { get; set; }
    }

    public class Amount
    {
        public string total { get; set; }
        public string currency { get; set; }
        public Details details { get; set; }
    }

    public class PaymentOptions
    {
        public string allowed_payment_method { get; set; }
    }

    public class Item
    {
        public string name { get; set; }
        public string description { get; set; }
        public string quantity { get; set; }
        public string price { get; set; }
        public string tax { get; set; }
        public string sku { get; set; }
        public string currency { get; set; }
    }

    public class ShippingAddress
    {
        public string recipient_name { get; set; }
        public string line1 { get; set; }
        public string line2 { get; set; }
        public string city { get; set; }
        public string country_code { get; set; }
        public string postal_code { get; set; }
        public string phone { get; set; }
        public string state { get; set; }
    }

    public class ItemList
    {
        public List<Item> items { get; set; }
        public ShippingAddress shipping_address { get; set; }
    }

    public class Transaction
    {
        public Amount amount { get; set; }
        public string description { get; set; }
        public string custom { get; set; }
        public string invoice_number { get; set; }
        public PaymentOptions payment_options { get; set; }
        public string soft_descriptor { get; set; }
        public ItemList item_list { get; set; }
    }

    public class RedirectUrls
    {
        public string return_url { get; set; }
        public string cancel_url { get; set; }
    }
}