add_action( 'wp_enqueue_scripts', 'child_enqueue_styles', 15 );
function addOneClickCheckoutBtn() {
    // 取得商品ID
    $current_product_id = get_the_ID();
    // 依據商品ID取得商品
    $product = wc_get_product( $current_product_id );
    // 取得結帳網址
    $checkout_url = WC()->cart->get_checkout_url();
    // 只在簡單商品中運行
    if( $product->is_type( 'simple' ) ){
        ?>
        <script>
            jQuery(function($) {
                   $(".custom-checkout-btn").on("click", function() 
                    {
                       $(this).attr("href", function() 
                        {
                            return this.href + '&quantity=' + $('input.qty').val();
                        });
                    });?>
            });
        </script>
        <style>
        .raiseup_one_click_checkout_btn {
            margin: 0px 1em !important;
        }
        </style>
        <?php
        echo '<a href="'.$checkout_url.'?add-to-cart='.$current_product_id.'" class="single_add_to_cart_button button raiseup_one_click_checkout_btn">直接結帳</a>';
    }
}
add_action( 'woocommerce_after_add_to_cart_button', 'addOneClickCheckoutBtn' );

        <style>
        .raiseup_one_click_checkout_btn {
            margin: 0px 1em !important;
        }
        </style>