a_cst.user.js

// ==UserScript==
// @name         Amazon - Copy Subtitle Text
// @description  Amazon - Copy Subtitle Text.
// @version      0.2
// @author       to
// @namespace    https://github.com/to
// @license      MIT
// 
// @noframes
// @match        https://www.amazon.co.jp/gp/video/*
// @icon         https://www.google.com/s2/favicons?sz=64&domain=amazon.co.jp
// 
// @grant        GM_setClipboard
// @grant        GM_registerMenuCommand
// ==/UserScript==

let subs = [];

const getSub = () => {
	const el = document.querySelector('.atvwebplayersdk-captions-text');
	return el ?
		el.innerHTML.replaceAll('<br>', ' ').replaceAll(/<.+?>/g, ' ').trim() : '';
};

setInterval(() => {
	const t = getSub();
	if (t && t !== subs[subs.length - 1])
		subs.push(t);
}, 1000);

document.addEventListener('keydown', e => {
	if (e.ctrlKey && e.shiftKey && e.code === 'KeyC') {
		const t = getSub();
		if (t) GM_setClipboard(t);
	}
});

GM_registerMenuCommand('Copy subs', () => {
	const n = +prompt('How many?');
	if (n > 0) GM_setClipboard(subs.slice(-n).join('\n'));
});