Use order customer note as observations on the DPD Portugal for WooCommerce labels

woo_dpd_portugal_shipment_data_comments.php

<?php
// Works on DPD Portugal for WooCommerce 14.0 and above
add_filter( 'woo_dpd_portugal_shipment_data', function( $shipment_data, $order_id ) {
	if ( empty( trim( $shipment_data['comments'] ) ) ) {
		$order = wc_get_order( $order_id );
		if ( $order ) {
			$shipment_data['comments'] = trim( $order->get_customer_note() );
		}
	}
	return $shipment_data;
}, 10, 2 );