EM Pro - Add Surcharge per ticket

function stonehenge_em_add_surcharge( $EM_Event, $EM_Booking, $post_validation ) {
    if( $post_validation ) {
        if( empty($EM_Booking->booking_meta['surcharges']) ) $EM_Booking->booking_meta['surcharges'] = array();
            $spaces     = $EM_Booking->spaces;
            $netto      = '5.00';
            $tax_rate   = $EM_Booking->booking_tax_rate;
            $tax_perc   = ($tax_rate/100);
            $bruto      = ($netto / (1 + $tax_perc));
            $total      = number_format( ($bruto * $spaces), 4);

            $EM_Booking->booking_meta['surcharges'][] = array(
                'name'      => sprintf('Vollkasko für %d Personen', $spaces),
                'desc'      => '',
                'type'      => '#',
                'amount'    => $total,
                'tax'       => 'pre'
            );
        }
    }
add_action('em_booking_add', 'stonehenge_em_add_surcharge', 11, 3);