Magento 2 Paypal Fee (charge) and life cycle

In magento 2 paypal is implemented with module Payal , in case if the customer choose payapl as method of payment M2 use Magento\Paypal\Model\Cart in place of \Magento\Payment\Model\Cart , it used for collect items and amount and validate the infos collected before sending to paypal ,

The model have more additional methods as (it extend from parent Magento\Paypal\Model\Cart) :
getAmounts() : Get shipping, tax, subtotal and discount amounts all together
_validate( ) : Check the line items and totals according to PayPal business logic limitations
_importItemsFromSalesModel() : Import items from sales model with workarounds for PayPal

Yeah after collecting the amounts and validate let's consider we choose Paypal express , the checkout redirect the function Paypal/Controller/Express/Start Action but inside this action it use the global Api class Model/Api/Nvp.php

Example of customize Payapl Fee :
First you need explore the complete process of how to add fee to order totals in magento2 in this link : http://magento.stackexchange.com/questions/92774/how-to-add-fee-to-order-totals-in-magento2
Ok you want add charge in case of the customer use paypal as method payment , we need add custom item to request sent to paypal and change the total amount sent (the class responsive is Nvp.php),
ok you need create plugin , go to Ibnab/PF/etc/di.xml or Ibnab/PF/etc/frontend/di.xml and write this lines :

 
<?xml version="1.0"?>
<!--
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Sales\Model\Order">
        <plugin name="update_payment_fee_order" type="Ibnab\PF\Plugin\UpdateFeeForOrder"/>
    </type>
    <type name="Magento\Paypal\Model\Cart">
        <plugin name="update_paypal_fee_order" type="Ibnab\PF\\Plugin\UpdateFeeForOrder"/>
    </type>
</config>

Here we modified to method getAllItems() of parent class \Magento\Payment\Model\Cart and getAmounts() of Magento\Paypal\Model\Cart
And After that create cretae the class Ibnab/PF/Plugin/UpdateFeeForOrder.php :

 
<?php
namespace Ibnab\PF\Plugin;
class UpdateFeeForOrder
{
    /**
     * @var \Magento\Quote\Model\QuoteFactory
     */
    protected $quote;
    protected $logger;
    protected $_checkoutSession;
    protected $_registry;
    const AMOUNT_Payment = 'payment_fee';
    const AMOUNT_SUBTOTAL = 'subtotal';
    public function __construct(
        \Magento\Quote\Model\Quote $quote,
        \Psr\Log\LoggerInterface $logger,
        \Magento\Checkout\Model\Session $checkoutSession,
        \Magento\Framework\Registry $registry
    ) {
        $this->quote = $quote;
        $this->logger = $logger;
        $this->_checkoutSession = $checkoutSession;
        $this->_registry = $registry;
    }
    /**
     * Get shipping, tax, subtotal and discount amounts all together
     *
     * @return array
     */
    public function afterGetAmounts($cart,$result)
    {
        $total = $result;
        $quote = $this->_checkoutSession->getQuote();
        $paymentMethod = $quote->getPayment()->getMethod();
        $paypalMehodList = ['payflowpro','payflow_link','payflow_advanced','braintree_paypal','paypal_express_bml','payflow_express_bml','payflow_express','paypal_express'];
        
        if(in_array($paymentMethod,$paypalMehodList)){
        $total[self::AMOUNT_SUBTOTAL] = $total[self::AMOUNT_SUBTOTAL] + $quote->getFeeAmount();
        
        }
        
        return  $total;
    }
    /**
     * Get shipping, tax, subtotal and discount amounts all together
     *
     * @return array
     */
    public function beforeGetAllItems($cart)
    {
        $paypalTest = $this->_registry->registry('is_paypal_items')? $this->_registry->registry('is_paypal_items') : 0;
        $quote = $this->_checkoutSession->getQuote();
        $paymentMethod = $quote->getPayment()->getMethod();
        
        $paypalMehodList = ['payflowpro','payflow_link','payflow_advanced','braintree_paypal','paypal_express_bml','payflow_express_bml','payflow_express','paypal_express'];
        if($paypalTest < 3 && in_array($paymentMethod,$paypalMehodList)){
        if(method_exists($cart , 'addCustomItem' ))
        {
        $cart->addCustomItem(__("Payment Fee"), 1 ,$quote->getFeeAmount());
        $reg = $this->_registry->registry('is_paypal_items');
        $current = $reg + 1 ;
        $this->_registry->unregister('is_paypal_items');
        $this->_registry->register('is_paypal_items', $current);
        }
        }
    }
}

Here we use the concept of after method in plugin it pass ed 2 Arguments the object of class and the result final , in afterGetAmounts we test if current payment method is paypal we add the charge ,
and in second method we used the concept of before and passed the object of class :
we have here 2 test for the current method and the number of access to method for ignoring the duplicate item .

You can get the complet Magento 2 Payment Fee support Paypal

Note: