/********************************************************************************************
*Add this on functions.php of your WordPress
*********************************************************************************************/

function add_content_after_addtocart() {

    // get the current post/product ID
    $current_product_id = get_the_ID();

    // get the product based on the ID
    $product = wc_get_product( $current_product_id );

    // get the "Checkout Page" URL
    $checkout_url = WC()->cart->get_checkout_url();

    // run only on simple products
    if( $product->is_type( 'simple' ) )
    ?>
        <script>
    jQuery(function($) {

        // if our custom button is clicked
        $(".custom-checkout-btn").on("click", function() {

            // get the value of the "href" attribute
            $(this).attr("href", function() {
                // return the "href" value + the string "&quantity=" + the current selected quantity number
                return this.href + '&quantity=' + $('input.qty').val();
            });

        });
    });
    </script>
    <?php

     {

        echo '<a href="'.$checkout_url.'?add-to-cart='.$current_product_id.'" class="single_add_to_cart_button button custom-checkout-btn alt">Buy Instant</a>';
    }
}
add_action( 'woocommerce_after_add_to_cart_button', 'add_content_after_addtocart' );