How to include custom meta data and custom fields to Woocommerce-pdf-invoices-packing-slips plugin.

wpo_wcpdf_add_meta_data_to_invoice.php

<?php

// hooks into invoice.php
add_action( 'wpo_wcpdf_after_order_data', 'msp_add_meta_data_to_wpo_invoice', 100, 2 );

function msp_add_meta_data_to_wpo_invoice( $type, $order ){
	/**
	 * Adds custom meta data to WPO generated invoice plugin
	 * DOC: https://docs.wpovernight.com/woocommerce-pdf-invoices-packing-slips/pdf-template-action-hooks/
	 * @param string $template_type
	 * @param WC_Order $order
	 */
	$id = $order->get_order_number();

  // Label => custom meta_key added to order
	$custom_fields = array(
		'AVS Result' => 'mes_avs_result',
		'Card Type' => 'mes_card_type',
		'Purchase Order' => '_billing_po'
	);

  // print non-empty fields
	foreach( $custom_fields as $label => $meta_key ){
		$meta_value = get_post_meta( $id, $meta_key, true );

		if( ! empty( $meta_value ) ){
			printf( "<p><strong>%s</strong>: %s</p>", $label, $meta_value );
		}
	}

}

?>