Magento – Add payment specific fee in Magento2

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;
?>

I found this article how to add custom fee and only added couple changes to collect() function, like this:

public function collect(Mage_Sales_Model_Quote_Address $address)
{
    parent::collect($address);
    try{
        $quote = $address->getQuote();
        $code = $quote->getPayment()->getMethodInstance()->getCode();
        if($code == 'my_payment'){
                $this->_setAmount(0);
                $this->_setBaseAmount(0);
                $items = $this->_getAddressItems($address);
                if (!count($items)) {
                    return $this; 
                }

                $exist_amount = $quote->getFeeAmount();
                //$fee = Excellence_Fee_Model_Fee::getFee(); 
                $fee=Mage::getStoreConfig('payment/my_payment/price');
                $balance = $fee - $exist_amount;
                $address->setFeeAmount($balance);
                $address->setBaseFeeAmount($balance);
                $quote->setFeeAmount($balance);
                $address->setGrandTotal($address->getGrandTotal() + $address->getFeeAmount());
                $address->setBaseGrandTotal($address->getBaseGrandTotal() + $address->getBaseFeeAmount());
        }
    } catch(Exception $e){}
}

Now only when user selects my payment method this fee would be added to totals