Add dynamic custom fee magento 2

Question

I am building a custom fee module for magento 2. I add a checkbox to the checkout page, if user ticks that checkbox, I will add a custom fee to the order. I successfully added the custom fee to order following this tutorial:

https://magento.stackexchange.com/questions/92774/how-to-add-fee-to-order-totals-in-magento-2

However, my custom fee is dynamic since it is fetched from external API. Let's take a look at:

app\code\Sugarcode\Test\Model\Total\Fee.php

<?php
/**
 * Copyright © 2015 Magento. All rights reserved.
 * See COPYING.txt for license details.
 */
namespace Sugarcode\Test\Model\Total;


class Fee extends \Magento\Quote\Model\Quote\Address\Total\AbstractTotal
{
   /**
     * Collect grand total address amount
     *
     * @param \Magento\Quote\Model\Quote $quote
     * @param \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment
     * @param \Magento\Quote\Model\Quote\Address\Total $total
     * @return $this
     */
    protected $quoteValidator = null; 

    public function __construct(\Magento\Quote\Model\QuoteValidator $quoteValidator)
    {
        $this->quoteValidator = $quoteValidator;
    }
  public function collect(
        \Magento\Quote\Model\Quote $quote,
        \Magento\Quote\Api\Data\ShippingAssignmentInterface $shippingAssignment,
        \Magento\Quote\Model\Quote\Address\Total $total
    ) {
        parent::collect($quote, $shippingAssignment, $total);


        $exist_amount = 0; //$quote->getFee(); 
        // $fee = 100; //Excellence_Fee_Model_Fee::getFee();
        // TAKE A LOOK AT THIS:
        $fee = getFeeFromExternalAPI()

        // I successfully get the FEE but how to update the value of checkbox in the UI???

        $balance = $fee - $exist_amount;
        
        $total->setTotalAmount('fee', $balance);
        $total->setBaseTotalAmount('fee', $balance);

        $total->setFee($balance);
        $total->setBaseFee($balance);

        $total->setGrandTotal($total->getGrandTotal() + $balance);
        $total->setBaseGrandTotal($total->getBaseGrandTotal() + $balance);


        return $this;
    } 

    protected function clearValues(Address\Total $total)
    {
        $total->setTotalAmount('subtotal', 0);
        $total->setBaseTotalAmount('subtotal', 0);
        $total->setTotalAmount('tax', 0);
        $total->setBaseTotalAmount('tax', 0);
        $total->setTotalAmount('discount_tax_compensation', 0);
        $total->setBaseTotalAmount('discount_tax_compensation', 0);
        $total->setTotalAmount('shipping_discount_tax_compensation', 0);
        $total->setBaseTotalAmount('shipping_discount_tax_compensation', 0);
        $total->setSubtotalInclTax(0);
        $total->setBaseSubtotalInclTax(0);
    }
    /**
     * @param \Magento\Quote\Model\Quote $quote
     * @param Address\Total $total
     * @return array|null
     */
    /**
     * Assign subtotal amount and label to address object
     *
     * @param \Magento\Quote\Model\Quote $quote
     * @param Address\Total $total
     * @return array
     * @SuppressWarnings(PHPMD.UnusedFormalParameter)
     */
    public function fetch(\Magento\Quote\Model\Quote $quote, \Magento\Quote\Model\Quote\Address\Total $total)
    {
        return [
            'code' => 'fee',
            'title' => 'Fee',
            'value' => 100
        ];
    }

    /**
     * Get Subtotal label
     *
     * @return \Magento\Framework\Phrase
     */
    public function getLabel()
    {
        return __('Fee');
    }

Please take a look at this function getFeeFromExternalAPI. Instead of using static fee (100$), I get the fee from external API. I got the fee BUT how to update the checkbox value in the UI (HTML). How can I notify the UI to change?

I am thinking of firing an event after I get the response successfully. And then I observe that event. However, the observer is PHP file. How can I update UI from PHP file