Here is a code snippet that you can use to display Webform pane values into your invoices.
Feel free to edit it to adjust your needs.

=============================================================
== Place the following into your theme/preprocess function ==
=============================================================
<?php 
  $webformPaneValues = array();

  $webforms = _uc_webform_pane_get_nodes();
  foreach ($webforms as $webform) {
    $components = array();
    $nid = $webform->nid;
    $node = node_load($nid);
    
    if(FALSE !== $node){
      foreach ($node->webform['components'] as $field){
	$components[] = array(
	  'name' => check_plain($field->name),
	  'value' => ${'order_webform_'.$nid.'_'.$field['form_key']},
	);
      }
      
      $webformPaneValues[] = array(
	'title' => check_plain($node->title),
	'components' => $components,
      );
    }
  }
  
  // Now, just pass $webformPaneValues to the template, for example through the template variables
  $tplVars = array(
    'your-variable-1' => $yourVariable1,
    'your-variable-2' => $yourVariable2,
    'webformPaneValues' => $webformPaneValues,
  );
  return theme('your-theme-hook', $tplVars)
?>

====================================================
== Place the following into your invoice template ==
====================================================
<?php if($webformPaneValues): ?>
  <table>
    <?php foreach($webformPaneValues as $webformPaneValue): ?>
      <tr>
	<td colspan="2" bgcolor="#006699">
	  <strong><?php print $webformPaneValue['title']; ?></strong>
	</td>
      </tr>
      <tr>
	<td colspan="2">
	  <table border="0" cellpadding="1" cellspacing="0" width="100%">
	    <?php foreach ($webformPaneValue['components'] as $component): ?>
	      <tr>
		<td nowrap="nowrap">
		  <strong><?php print $component['name']; ?>:</strong>
		</td>
		<td width="98%">
		  <?php print $component['value'] ?>
		</td>
	      </tr>  
	    <?php endforeach; ?>
	  </table>
	</td>
      </tr>
    <?php endforeach; ?>
  </table>
<?php endif; ?>