Add Shipping Methods Tab To Companies.update In Vendor Panel?

Hello, I want vendors to be able to select their own shipping methods through the companies.update 'Shipping methods' tab.

I was able to make the tab appear by changing app/controllers/backend/companies.php from this:

    if (!Registry::get('runtime.company_id')) {
        $shippings = db_get_hash_array(
            "SELECT a.shipping_id, a.status, b.shipping"
            . " FROM ?:shippings as a"
            . " LEFT JOIN ?:shipping_descriptions as b ON a.shipping_id = b.shipping_id AND b.lang_code = ?s"
            . " WHERE a.company_id = 0 AND a.status = 'A'"
            . " ORDER BY a.position",
            'shipping_id', DESCR_SL
        Tygh::$app['view']->assign('shippings', $shippings);
    if (!fn_allowed_for('ULTIMATE')) {
        $tabs['shipping_methods'] = array(
            'title' => __('shipping_methods'),
            'js' => true

to just a different if condition in the first line:

if (fn_allowed_for('MULTIVENDOR'))

So I got it to appear in the vendor panel, but it's not functional, meaning that if I check or uncheck a shipping method in the vendor panel, then it doesn't save for some reason. Why is this, and how can I fix it?

Your solution works on my local installation. I can enable and disable shipping methods under vendor account. Try to clear cache