{
  "manifest_version": 2,

  "name": "Getting started example",
  "description": "This extension shows a Google Image search result for the current page",
  "version": "1.0",

  "browser_action": {
    "default_title": "ForX",
    "default_icon": "icon.png"
    // "default_popup": "popup.html"
  },
  // "content_scripts": [
  //   {
  //     "matches": ["https://www.amazon.com/gp/buy/payselect/handlers/*"],
  //     "js": ["jquery-2.1.4.min.js", "injected.js"],
  //     "run_at": "document_end"
  //   }
  // ],
  "background": {
    "scripts": ["injected.js", "content_scripts.js"],
    "persistent": false
  },
  "permissions": [
    // "tabs",
    "activeTab",
    "https://ajax.googleapis.com/"
    // "https://www.amazon.com/gp/buy/payselect/handlers/*"
  ]
}

{
  "manifest_version": 2,

  "name": "Getting started example",
  "description": "This extension shows a Google Image search result for the current page",
  "version": "1.0",

  // "browser_action": {
  //   "default_title": "ForX",
  //   "default_icon": "icon.png"
  //   // "default_popup": "popup.html"
  // },
  "content_scripts": [
    {
      "matches": [
        "https://www.amazon.com/gp/buy/payselect/handlers/*",
        "https://www.amazon.com/gp/buy/spc/handlers/*",
        "https://www.google.com/*"
      ],
      "js": ["jquery-2.1.4.min.js", "injected.js", "content_scripts.js"],
      "css": ["styles.css"],
      "run_at": "document_end"
    }
  ],
  // "background": {
  //   "scripts": ["injected.js", "content_scripts.js"],
  //   "persistent": false
  // },
  "permissions": [
    // "tabs",
    // "activeTab",
    // "https://ajax.googleapis.com/"
    // "https://www.amazon.com/gp/buy/payselect/handlers/*"
  ]
}

Use content scripts to auto load on checkout
Use content scripts to auto load on checkout
