Use a different GLS contact id based on shipment data

functions.php

<?php

add_filter( 'woocommerce_gzd_gls_label_api_request', function( $request, $label ) {
    if ( $shipment = $label->get_shipment() ) {
        $country = $shipment->get_country();
        
        if ( 'AT' === $country ) {
            $request['Shipment']['Shipper']['ContactID'] = '12345';
        }
    }
    
    return $request;
}, 10, 2 );