Edits order details page on the fly requires tampermonkey

fckcustoms.js

// ==UserScript==
// @name         fckcustoms
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  Customs are literally scammers
// @author       You
// @require      http://code.jquery.com/jquery-3.4.1.min.js
// @match        https://www.amazon.com/gp/your-account/order-details*
// @run-at document-body
// @grant        none
// ==/UserScript==

(function() {
    'use strict';


/*setMutationHandler(document, '#od-subtotals', function(nodes) {
    this.disconnect();
    nodes.forEach(function(n) {

    });
});*/
    (new MutationObserver(check)).observe(document, {childList: true, attributes: true, characterData: true, subtree: true,});

function check(changes, observer) {
    if(document.querySelector('#od-subtotals')) {
        observer.disconnect();
//Shipping price
      // document.querySelector('#od-subtotals > .a-row:nth-child(3) .a-column:nth-child(2) > .a-color-base').innerHTML = "$15"
       //coupon savings
        $('#od-subtotals > .a-row:nth-child(3)').append(`<div class="a-row">
<div class="a-column a-span7 a-text-left">
        <span class="a-color-base">
            Promotion Applied:
        </span>
</div>
<div class="a-column a-span5 a-text-right a-span-last">
    <span class="a-color-base">
        $-100.00
    </span>
</div>
</div>`)
       
var sub = parseFloat(document.querySelector("#od-subtotals > .a-row:nth-child(2) .a-column:nth-child(2) > .a-color-base").innerText.replace(/[^\d.]/g, ''))
var ship =parseFloat(document.querySelector("#od-subtotals > .a-row:nth-child(3) .a-column:nth-child(2) > .a-color-base").innerText.replace(/[^\d.]/g, ''))
 var coupon = parseFloat(-Math.abs(document.querySelector("#od-subtotals > .a-row:nth-child(3) > .a-row .a-column:nth-child(2) > .a-color-base").innerText.replace(/[^\d.]/g, '')))
 var totalb = ship + sub + coupon;
    document.querySelector("#od-subtotals > .a-row:nth-child(5) .a-column:nth-child(2) > .a-color-base").innerText = "$" + totalb
   var tax = parseFloat(document.querySelector("#od-subtotals > .a-row:nth-child(6) .a-column:nth-child(2) > .a-color-base").innerText.replace(/[^\d.]/g, ''))
   var grandTotal = totalb+tax
       document.querySelector("#od-subtotals > .a-row:nth-child(8) .a-column:nth-child(2) > .a-color-base").innerText = "$" + grandTotal
    }
}
})();