“wpcfe_shipment_table_header_action” an action hook is use to add additional column in the shipment table header. Note: This hook will add column after action column.

Copy and paste the following to for customized the label in the theme functions.php file

// Add additional header "Custom Data"
add_action( 'wpcfe_shipment_table_header_action', function(){
    ?>
    <th class="no-space">Custom Action</th>
    <?php
}, 25 );