Line stickers save as , badly written , you can see my random comments!, but it works demo:https://gfycat.com/MinorInconsequentialBetafish

saveStickersAs.user.js

// ==UserScript==
// @name         Save stickers zip Userscript
// @namespace    http://tampermonkey.net/
// @version      0.1
// @description  add sticker link next to gift or see other items button
// @author       Modder4869
// @match        https://store.line.me/stickershop/product/*
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    //declare product id
//declare product id
var productID;
var stickerZipName;

// get product id from current link
    productID=window.location.href.split('/product')[1].split('/')[1]

    // declare download link

   // var downloadLink = `https://stickershop.line-scdn.net/stickershop/v1/product/${productID}/android/stickers.zip`
    var downloadLink = `https://stickershop.line-scdn.net/stickershop/v1/product/${productID}/iphone/stickers@2x.zip`
    var copyButtonHtml=`<a href=${downloadLink}>Right click then save as to download sticker</a>`;

    //log downloadLink
console.log(downloadLink);

//find send as gift  or see other items button
var sendAsGift = document.querySelector('.MdBtn01P02');
var seeOthersItems = document.querySelector('.mdMN05Btn');


//clone save as gif button or other items if it doesn't exist
if(seeOthersItems){
var cloneCopyButton = seeOthersItems.cloneNode(true)
document.querySelector('.mdMN05Btn').appendChild(cloneCopyButton)
}
else if (sendAsGift){
    cloneCopyButton = sendAsGift.cloneNode(true)
    document.querySelector('.mdCMN38Item01').appendChild(cloneCopyButton)
}
cloneCopyButton.outerHTML=copyButtonHtml
 cloneCopyButton.innerText = "CopyLink"
console.log(cloneCopyButton)
// function getStickerZipURL(){
//     //copy sticker downloadLink to clipboard
//     copyToClipboard(downloadLink);
// }
//find better way to add button
// ......

//copy to clipboard function
// function copyToClipboard (str){
//     let el = document.createElement('textarea');
//     el.value = str;
//     document.body.appendChild(el);
//     el.select();
//     document.execCommand('copy');
//     document.body.removeChild(el);
// };
function downloadFileLink(){
    // var fileName = 'downloadLink.zip';
    var link = document.createElement("a");
    //source https://stackoverflow.com/questions/27115748/download-attribute-not-working-in-firefox
    if (link.download !== undefined) { // feature detection
        // Browsers that support HTML5 download attribute
        link.setAttribute("href", downloadLink);

        // link.setAttribute("download", fileName);
        // link.setAttribute("target", "_blank");
        // link.download=fileName
        document.body.appendChild(link);
        link.click();
        document.body.removeChild(link);
    } else {
        alert('Stickers download only works in Chrome, Firefox, and Opera.');
    }
}


})();