<?php

/**
 * @snippet       WooCommerce - Sets custom reference in Unifaun
 * @author        Redlight Media AB / Christopher Hedqvist
 * @compatible    WooCommerce 3.7.0
 */
function redlight_unifaun_reference( $shipment, $order ) {
    $shipment['senderReference'] = 'TOP-' . $order->get_order_number();

    return $shipment;
}
add_filter('ac_wc_unifaun_shipment_data', 'redlight_unifaun_reference', 10, 2 );