Magento 2 – How to Add Additional Charge on COD

You referral link can only remove the code from payment list at check payment step.If you set error message at session (Mage::getSingleton('core/session')->addError(Mage::helper('payment')->__('Code is not avaliable.'))) on function isAvaliable it can be shown on next step.

According my concept it will be good idea to do something at methods.phtml where all payment method has been listing during the checkout.

<?php
$isCodeAvalibale=0;
    foreach ($this->getMethods() as $_method):
        $_code = $_method->getCode();
    if($_code =='cashondelivery'):
    $isCodeAvalibale=1;
    endif;
    endforeach;
if($isCodeAvalibale==0):
echo Mage::helper('payment')->__('Code is not avaliable.'));
endif;
?>

In your custom module :

di.xml

    <config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
          <type name="Magento\OfflinePayments\Model\Cashondelivery">
               <plugin sortOrder="1" name="restrictByCustomer"
            type="[vendor]\[module]\Plugin\Payment\Method\CashOnDelivery\Available"/>
          </type>
    </config>

[vendor][module]\Plugin\Payment\Method\CashOnDelivery\Available.php

<?php
/**
 * Copyright © 2016 MageWorx. All rights reserved.
 * See LICENSE.txt for license details.
 */

namespace [vendor]\[module]\Plugin\Payment\Method\CashOnDelivery;

use Magento\Customer\Model\Session as CustomerSession;
use Magento\Backend\Model\Auth\Session as BackendSession;
use Magento\OfflinePayments\Model\Cashondelivery;

class Available
{

    /**
     * @var CustomerSession
     */
    protected $customerSession;

    /**
     * @var BackendSession
     */
    protected $backendSession;

    /**
     * @param CustomerSession $customerSession
     * @param BackendSession $backendSession
     */
    public function __construct(
    CustomerSession $customerSession,
    BackendSession $backendSession
    ) {
    $this->customerSession = $customerSession;
    $this->backendSession = $backendSession;
    }

    /**
     *
     * @param Cashondelivery $subject
     * @param $result
     * @return bool
     * @throws \Magento\Framework\Exception\LocalizedException
     */
    public function afterIsAvailable(Cashondelivery $subject, $result)
    {
    // Do not remove payment method for admin
    if ($this->backendSession->isLoggedIn()) {
        return $result;
    }
    ..........................        
    return true/false;

    }

In this some time you will get the JS error of :

window.checkoutConfig.payment.instructions

In this you need to change the .js file of that payment method and remove this line.