Amazon.com_shortlink.user.js

// ==UserScript==
// @name        Amazon.com shortlink
// @namespace   http://ajitek.net/gm/amazon-shortlink
// @description Add <link rel="shortlink" ...> for Amazon.com pages
// @include     http://amazon.com/*/dp/*
// @include     http://www.amazon.com/*/dp/*
// @include     https://amazon.com/*/dp/*
// @include     https://www.amazon.com/*/dp/*
// @include     http://amazon.com/*/e/*
// @include     http://www.amazon.com/*/e/*
// @include     https://amazon.com/*/e/*
// @include     https://www.amazon.com/*/e/*
// @version     1
// @grant       none
// ==/UserScript==

window.addEventListener('load', function (e) {
  document.head.appendChild((function () {
    var shortlink = document.createElement('link');
    shortlink.setAttribute('rel', 'shortlink');
    shortlink.setAttribute('href', (function () {
      var link_parts = (function () {
        var links = document.getElementsByTagName('link');
        for (i in links) {
          if (links[i].getAttribute('rel') == 'canonical')
          return links[i].getAttribute('href');
        }
        return '';
      }) ().split('/');
      var kind = link_parts[link_parts.length - 2];
      var asin = link_parts[link_parts.length - 1];
      var scheme = link_parts[0];
      return scheme + '//amazon.com/-/' + kind + '/' + asin;
    }) ());
    return shortlink;
  }) ())
});