functions.php

<?php
/**
 * @snippet       WooCommerce - Adds ADDON to Shipment
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 6.0.0
 */
function redlight_unifaun_dhl_addon_spbag( $shipment, $order ) {
    // Only ASPO - DHL Service Point 
    if( 'ASPO' === $shipment['service']['id'] ){
        $shipment['service']['addons'][] = array(
            'id' => 'SPBAG'
        );
    }
  return $shipment;
}
add_filter('ac_wc_unifaun_shipment_data', 'redlight_unifaun_dhl_addon_spbag', 10, 2 );