Point-1: 不清楚你在哪里$order_id 从…起我想是在Shortcode.

Point-2: 你不需要这么做。使用ob_* 返回函数Shortcode 替换内容，只需返回即可。

假设代码的其他部分正确，下面是可能的代码：

如果在主题的functions.php 文件：

/**
 * Add Shortcode 
 */
function tracking_shortcode( $attrs ) {
    $attrs = shortcode_atts( array(
        \'order_id\' => \'\'
    ), $attrs );
    if( empty( $attrs[\'order_id\'] ) ) return \'\';

    $order = new WC_Order( $attrs[\'order_id\'] );
    // I\'ve followed your code, but may be even the above line is not needed.
    // may be you can just use $attrs[\'order_id\'] directly to get tracking code
    return get_post_meta( $order->id, \'_shipping_tracking_code\', true );
}
add_shortcode( \'tracking_code\', \'tracking_shortcode\' );

/**
 * Add Shortcode 
 */
function tracking_shortcode( $attrs ) {
    $attrs = shortcode_atts( array(
        \'order_id\' => \'\'
    ), $attrs );
    if( empty( $attrs[\'order_id\'] ) ) return \'\';

    $order = new WC_Order( $attrs[\'order_id\'] );
    return get_post_meta( $order->id, \'_shipping_tracking_code\', true );
}

/**
 * Initiate Shortcode 
 */
function tracking_shortcode_init()
{
    add_shortcode( \'tracking_code\', \'tracking_shortcode\' );
}
add_action(\'init\', \'tracking_shortcode_init\');