Add a custom field (in an order) to the emails
add_filter( 'woocommerce_email_order_meta_fields', 'custom_woocommerce_email_order_meta_fields', 10, 3 );
function custom_woocommerce_email_order_meta_fields( $fields, $sent_to_admin, $order ) {
$fields['meta_key'] = array(
'label' => __( 'Label' ),
'value' => get_post_meta( $order->id, 'meta_key', true ),
);
return $fields;
}