sub invoice {
   my $invoice_data = shift;

   my $items_html = invoice_items($invoice_data->{items});

   my $invoice_number = $invoice_data->{number};

   my $html = <<HTML;
<div class="invoice">
  <div class="invoice_no">$invoice_number</div>
  <div class="invoice_items">
       $items_html
  </div><!-- END invoice_items -->
</div><!-- END invoice -->
HTML
   
 return $html;
}