Get error on checkout page in Payment Method Section for my Payment Plugin "The method or operation is not implemented."

using Nop.Core;
using Nop.Core.Domain.Directory;
using Nop.Core.Domain.Orders;
using Nop.Core.Domain.Payments;
using Nop.Core.Domain.Shipping;
using Nop.Core.Plugins;
using Nop.Services.Configuration;
using Nop.Services.Directory;
using Nop.Services.Localization;
using Nop.Services.Orders;
using Nop.Services.Payments;
using Nop.Services.Tax;
using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Web;
using System.Web.Mvc;
using System.Web.Routing;

namespace Nop.Plugin.Payments.SamanPay
{
    public class SamanPayProcessor : BasePlugin, IPaymentMethod
    {
        #region Fields

        private readonly SamanPaySettings _samanPayPluginSettings;
        private readonly ISettingService _settingService;
        private readonly IOrderTotalCalculationService _orderTotalCalculationService;
        private readonly CurrencySettings _currencySettings;
        private readonly IWebHelper _webHelper;
        private readonly ICheckoutAttributeParser _checkoutAttributeParser;
        private readonly ITaxService _taxService;
        private readonly HttpContextBase _httpContext;
        private readonly ICurrencyService _currencyService;


        #endregion

        #region Ctor

        public SamanPayProcessor(SamanPaySettings SamanPayPluginSettings,
            ISettingService settingService, ICurrencyService currencyService,
            CurrencySettings currencySettings, IWebHelper webHelper,
            ICheckoutAttributeParser checkoutAttributeParser, ITaxService taxService,
            IOrderTotalCalculationService orderTotalCalculationService, HttpContextBase httpContext)
        {
            this._samanPayPluginSettings = SamanPayPluginSettings;
            this._settingService = settingService;
            this._currencyService = currencyService;
            this._currencySettings = currencySettings;
            this._webHelper = webHelper;
            this._checkoutAttributeParser = checkoutAttributeParser;
            this._taxService = taxService;
            this._orderTotalCalculationService = orderTotalCalculationService;
            this._httpContext = httpContext;
        }

        #endregion

        #region Utilities

        /// <summary>
        /// Gets Paypal URL
        /// </summary>
        /// <returns></returns>
        private string GetSamanPayUrl()
        {
            return "https://sep.shaparak.ir/Payment.aspx";
        }

        #endregion

        #region Methods

        /// <summary>
        /// Process a payment
        /// </summary>
        /// <param name="processPaymentRequest">Payment info required for an order processing</param>
        /// <returns>Process payment result</returns>
        public ProcessPaymentResult ProcessPayment(ProcessPaymentRequest processPaymentRequest)
        {
            var result = new ProcessPaymentResult();
            result.NewPaymentStatus = PaymentStatus.Pending;
            return result;
        }

        /// <summary>
        /// Post process payment (used by payment gateways that require redirecting to a third-party URL)
        /// </summary>
        /// <param name="postProcessPaymentRequest">Payment info required for an order processing</param>
        public void PostProcessPayment(PostProcessPaymentRequest postProcessPaymentRequest)
        {
            var builder = new StringBuilder();

            builder.Append(GetSamanPayUrl());

            var orderTotal = Math.Round(postProcessPaymentRequest.Order.OrderTotal, 2);
            builder.AppendFormat("&Amount={0}", orderTotal.ToString("0.00", CultureInfo.InvariantCulture));

            builder.AppendFormat("&MID={0}", _samanPayPluginSettings.MerchantID);

            builder.AppendFormat("&ResNum={0}", postProcessPaymentRequest.Order.Id);

            string returnUrl = _webHelper.GetStoreLocation(false) + "Plugins/SamanPay/ResultHandler";
            builder.AppendFormat("&RedirectURL={0}", HttpUtility.UrlEncode(returnUrl));

            _httpContext.Response.Redirect(builder.ToString());
        }

        /// <summary>
        /// Returns a value indicating whether payment method should be hidden during checkout
        /// </summary>
        /// <param name="cart">Shoping cart</param>
        /// <returns>true - hide; false - display.</returns>
        public bool HidePaymentMethod(IList<ShoppingCartItem> cart)
        {
            //you can put any logic here
            //for example, hide this payment method if all products in the cart are downloadable
            //or hide this payment method if current customer is from certain country
            return false;
        }

        /// <summary>
        /// Captures payment
        /// </summary>
        /// <param name="capturePaymentRequest">Capture payment request</param>
        /// <returns>Capture payment result</returns>
        public CapturePaymentResult Capture(CapturePaymentRequest capturePaymentRequest)
        {
            var result = new CapturePaymentResult();
            result.AddError("Capture method not supported");
            return result;
        }

        /// <summary>
        /// Refunds a payment
        /// </summary>
        /// <param name="refundPaymentRequest">Request</param>
        /// <returns>Result</returns>
        public RefundPaymentResult Refund(RefundPaymentRequest refundPaymentRequest)
        {
            var result = new RefundPaymentResult();
            result.AddError("Refund method not supported");
            return result;
        }

        /// <summary>
        /// Voids a payment
        /// </summary>
        /// <param name="voidPaymentRequest">Request</param>
        /// <returns>Result</returns>
        public VoidPaymentResult Void(VoidPaymentRequest voidPaymentRequest)
        {
            var result = new VoidPaymentResult();
            result.AddError("Void method not supported");
            return result;
        }

        /// <summary>
        /// Process recurring payment
        /// </summary>
        /// <param name="processPaymentRequest">Payment info required for an order processing</param>
        /// <returns>Process payment result</returns>
        public ProcessPaymentResult ProcessRecurringPayment(ProcessPaymentRequest processPaymentRequest)
        {
            var result = new ProcessPaymentResult();
            result.AddError("Recurring payment not supported");
            return result;
        }

        /// <summary>
        /// Cancels a recurring payment
        /// </summary>
        /// <param name="cancelPaymentRequest">Request</param>
        /// <returns>Result</returns>
        public CancelRecurringPaymentResult CancelRecurringPayment(CancelRecurringPaymentRequest cancelPaymentRequest)
        {
            var result = new CancelRecurringPaymentResult();
            result.AddError("Recurring payment not supported");
            return result;
        }

        /// <summary>
        /// Gets a value indicating whether customers can complete a payment after order is placed but not completed (for redirection payment methods)
        /// </summary>
        /// <param name="order">Order</param>
        /// <returns>Result</returns>
        public bool CanRePostProcessPayment(Order order)
        {
            if (order == null)
                throw new ArgumentNullException("order");

            //let's ensure that at least 5 seconds passed after order is placed
            //P.S. there's no any particular reason for that. we just do it
            if ((DateTime.UtcNow - order.CreatedOnUtc).TotalSeconds < 5)
                return false;

            return true;
        }

        /// <summary>
        /// Gets a route for provider configuration
        /// </summary>
        /// <param name="actionName">Action name</param>
        /// <param name="controllerName">Controller name</param>
        /// <param name="routeValues">Route values</param>
        public void GetConfigurationRoute(out string actionName, out string controllerName, out RouteValueDictionary routeValues)
        {
            actionName = "Configure";
            controllerName = "SamanPay";
            routeValues = new RouteValueDictionary { { "Namespaces", "Nop.Plugin.Payments.SamanPay.Controllers" }, { "area", null } };
        }

        /// <summary>
        /// Gets a route for payment info
        /// </summary>
        /// <param name="actionName">Action name</param>
        /// <param name="controllerName">Controller name</param>
        /// <param name="routeValues">Route values</param>
        public void GetPaymentInfoRoute(out string actionName, out string controllerName, out RouteValueDictionary routeValues)
        {
            actionName = "PaymentInfo";
            controllerName = "SamanPay";
            routeValues = new RouteValueDictionary { { "Namespaces", "Nop.Plugin.Payments.SamanPay.Controllers" }, { "area", null } };
        }

        public Type GetControllerType()
        {
            return typeof(SamanPay.Controllers.SamanPayController);
        }

        public decimal GetAdditionalHandlingFee(IList<ShoppingCartItem> cart)
        {
            throw new NotImplementedException();
        }

        public override void Install()
        {
            //settings
            var settings = new SamanPaySettings
            {
                MerchantID = 0,                
            };
            _settingService.SaveSetting(settings);

            //locales

            this.AddOrUpdatePluginLocaleResource("Plugins.Payments.SamanPay.Fields.RedirectionTip", "You will be redirected to SamanPay site to complete the order.");

            this.AddOrUpdatePluginLocaleResource("Plugins.Payments.SamanPay.Fields.MerchantID", "Merchant ID");
            this.AddOrUpdatePluginLocaleResource("Plugins.Payments.SamanPay.Fields.MerchantID.Hint", "Merchant ID");

            base.Install();
        }

        public override void Uninstall()
        {
            //settings
            _settingService.DeleteSetting<SamanPaySettings>();

            //locales

            this.DeletePluginLocaleResource("Plugins.Payments.SamanPay.Fields.RedirectionTip");

            this.DeletePluginLocaleResource("Plugins.Payments.SamanPay.Fields.MerchantID");
            this.DeletePluginLocaleResource("Plugins.Payments.SamanPay.Fields.MerchantID.Hint");

         
            base.Uninstall();
        }



        #endregion

        #region Properies

        /// <summary>
        /// Gets a value indicating whether capture is supported
        /// </summary>
        public bool SupportCapture
        {
            get
            {
                return false;
            }
        }

        /// <summary>
        /// Gets a value indicating whether partial refund is supported
        /// </summary>
        public bool SupportPartiallyRefund
        {
            get
            {
                return false;
            }
        }

        /// <summary>
        /// Gets a value indicating whether refund is supported
        /// </summary>
        public bool SupportRefund
        {
            get
            {
                return false;
            }
        }

        /// <summary>
        /// Gets a value indicating whether void is supported
        /// </summary>
        public bool SupportVoid
        {
            get
            {
                return false;
            }
        }

        /// <summary>
        /// Gets a recurring payment type of payment method
        /// </summary>
        public RecurringPaymentType RecurringPaymentType
        {
            get
            {
                return RecurringPaymentType.NotSupported;
            }
        }

        /// <summary>
        /// Gets a payment method type
        /// </summary>
        public PaymentMethodType PaymentMethodType
        {
            get
            {
                return PaymentMethodType.Redirection;
            }
        }

        /// <summary>
        /// Gets a value indicating whether we should display a payment information page for this plugin
        /// </summary>
        public bool SkipPaymentInfo
        {
            get
            {
                return false;
            }
        }

        #endregion

    }
}

System.NotImplementedException: The method or operation is not implemented. at Nop.Plugin.Payments.SamanPay.SamanPayProcessor.GetAdditionalHandlingFee(IList`1 cart) at Nop.Services.Payments.PaymentService.GetAdditionalHandlingFee(IList`1 cart, String paymentMethodSystemName) at Nop.Web.Controllers.CheckoutController.PreparePaymentMethodModel(IList`1 cart, Int32 filterByCountryId) at Nop.Web.Controllers.CheckoutController.OpcLoadStepAfterShippingMethod(List`1 cart) at Nop.Web.Controllers.CheckoutController.OpcSaveShippingMethod(FormCollection form)

        public decimal GetAdditionalHandlingFee(IList<ShoppingCartItem> cart)
        {
            throw new NotImplementedException();
        }

        public decimal GetAdditionalHandlingFee(IList<ShoppingCartItem> cart)
        {
            throw new NotImplementedException();
        }

        public decimal GetAdditionalHandlingFee(IList<ShoppingCartItem> cart)
        {
            return decimal.zero;
        }