<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
   <event name="payment_cart_collect_items_and_amounts">
        <observer name="add_fee_to_paypal" instance="VendorExtensionObserverAddFeeToPaypal" />
    </event>
</config>

<?php
namespace VendorExtensionObserver;

use MagentoFrameworkEventObserverInterface;
use MagentoFrameworkEventObserver;
use MagentoCheckoutModelSession;

class AddFeeToPaypal implements ObserverInterface
{
    public $checkout;

    public function __construct(Session $checkout)
    {
        $this->checkout = $checkout;
    }

    public function execute(Observer $observer)
    {
        $cart = $observer->getEvent()->getCart();
        $quote = $this->checkout->getQuote();
        $address = $quote->getIsVirtual() ? $quote->getBillingAddress() : $quote->getShippingAddress();

        if ($fee = $address->getCustomFee())
        {
            $cart->addCustomItem('Paypal Payment Fee', 1, $fee);
        }

        return $this;
    }
}