how to debug shipping module / class

Yes, add this function to your module

function my_print_r( $msg, $name="" )
{
    $error_dir="/path/to/wordpress/wp-content/debug.log";
    $msg = print_r( $msg, true );
    $log = $name . "  |  " . $msg . "\n";
    error_log( $log, 3, $error_dir );
}

And call my_print_r( $variable_to_inspect, 'description' );. The file debug.log is originally used by WP_DEBUG_LOG, and if you don’t have it enabled only your debug info will be there. This plugin of mine shows the content of that file in the dashboard.