Thank you so much for your help.
I have added this at line 30, it seems to be working fine.

<!-- Invoice -->
<div class="content forward"><?php echo '<a href="' . FILENAME_INVOICE . '.php?oID=' . $history['orders_id'] . '">' . zen_image_button(BUTTON_IMAGE_INVOICE_SMALL, BUTTON_INVOICE_SMALL_ALT) . '</a>'; ?></div>

A few more minor tweaks to the site and it will be ready to go live.