1. I followed this per the plugin note: Want to use your own template? Copy all the files from wp-content/plugins/woocommerce-pdf-invoices-packing-slips/templates/Simple to your (child) theme in wp-content/themes/pro-child/woocommerce/pdf/yourtemplate to customize them
and created a new template called SimpleNew
2. I edited the invoice.php file
//replaced:
<tr class=”payment-method”>
<th><?php _e( ‘Payment Method:’, ‘woocommerce-pdf-invoices-packing-slips’ ); ?></th>
<td><?php $this->payment_method(); ?></td>
</tr>
//with:
<tr class=”payment-method”>
<th><?php _e( ‘Payment Method:’, ‘woocommerce-pdf-invoices-packing-slips’ ); ?></th>
<td><?php $this->payment_method(); ?></td>
<th><?php _e( ‘Transaction ID:’, ‘wpo_wcpdf’ ); ?></th>
<td><?php $wpo_wcpdf->custom_field(‘Transaction ID’); ?><?php echo $this->order->get_transaction_id(); ?></td>
</tr>
But it didn’t work!