use an if statement to check the query string:
<?php if(isset($_GET['ckbox']) && $_GET['ckbox'] != "") { ?>
insert code here
<?php } ?>
and change the forms action:
action="<?php echo(htmlentities($_SERVER["PHP_SELF"], ENT_QUOTES)."?".$_SERVER["QUERY_STRING"]); ?>"
to:
action="<?php echo(htmlentities($_SERVER["PHP_SELF"], ENT_QUOTES)."?".(isset($_GET['ItineraryID'])?"ItineraryId=".$_GET['ItineraryID']:""); ?>"