userscripts/simplify.user.js

// ==UserScript==
// @name         Simplify URLs
// @version      0.1
// @description  Shortens URL for various websites
// @author       min
// @license      MIT; https://gogs.infcof.com/min/userscripts/src/master/LICENSE
// @updateURL    https://gogs.infcof.com/min/userscripts/raw/master/simplify.user.js
// @match        https://www.ebay.co.uk/itm/*
// @match        https://www.amazon.co.uk/*/dp/*
// @match        https://www.amazon.co.uk/gp/product/*
// @match        https://www.aliexpress.com/item/*
// @grant        none
// ==/UserScript==

(function() {
  'use strict';
  //let domain = window.location.protocol + '//' + window.location.hostname;
  let redir;
  if(window.location.hostname.indexOf("ebay") !== -1) {
    let s = window.location.pathname.split('/');
    redir = '/itm/' + s[s.length -1];
  }

  else if(window.location.hostname.indexOf("amazon") !== -1) {
    let s = window.location.pathname.split('/');
    redir = '/dp/' + s[s.length -2];
  }

  else if(window.location.hostname.indexOf("aliexpress") !== -1) {
    let s = window.location.pathname.split('/');
    redir = '/item//' + s[s.length -1];
  }

  if(window.location.pathname === redir) return;
  console.log("Shorting URL:", window.location.href, "->", redir);
  history.pushState(null, '', redir);

})();