Is there a a field in the cart you can check to identify if something is a contact lens? I imagine you have to loop through the cart and identify if a contact lens is present, and then add the "required" attribute to the fields if it is.
Form Elements added to this page should automatically be added to the persistent form. Please attach a copy of pp_confirm.php... you may need to adjust one line to maintain the new fields in the case of paypal express checkout.