<?php
// add custom field to invoice email
add_action( 'woocommerce_email_after_order_table', 'woocommerce_custom_invoice_fields' );
function woocommerce_custom_invoice_fields( $order ) {
?>
<p><strong><?php _e('Free Book:', 'woocommerce'); ?></strong> <?php echo get_post_meta( $order->id, 'Free Book', true ); ?></p>
<p><strong><?php _e('Free DVD:', 'woocommerce'); ?></strong> <?php echo get_post_meta( $order->id, 'Free DVD', true ); ?></p>
<p><strong><?php _e('Gift:', 'woocommerce'); ?></strong> <?php echo get_post_meta( $order->id, 'Gift Order', true ); ?></p>
<p><strong><?php _e('Gift Message:', 'woocommerce'); ?></strong> <?php echo get_post_meta( $order->id, 'Special Gift Message', true ); ?></p>
<?php
}
?>

Quelle: https://gist.github.com/kloon/4553982