amazon-url-normalizer.user.js

// ==UserScript==
// @name        Amazon URL normalizer
// @namespace   https://piou.dev
// @include     https://www.amazon.co.jp/*
// @version     1
// @grant       none
// @run-at      document-start
// ==/UserScript==

const regex = /^https:\/\/www.amazon.co.jp\/(?:.+\/)?(?:dp|gp\/product)\/([a-zA-Z0-9]+?)(?:[?\/].*|$)/
const id = location.href.match(regex)?.[1]

if(id !== null){
  history.replaceState(history.state, document.title, `https://www.amazon.co.jp/dp/${id}`)
}

document.addEventListener('DOMContentLoaded', () => {
  if(id !==null){
    const iframe = document.createElement('iframe')
    iframe.src = `https://keepa.com/iframe_addon.html#5-0-${id}`
    iframe.style = 'width: 100%; height: 491px; border: 0 none'
    
    const prevElement = document.getElementById('hover-zoom-end')
    if(prevElement){
      prevElement.parentNode.insertBefore(iframe, prevElement.nextElementSibling)
    }
  }
})