Content module problem

-- HikaShop version -- : 3.5
-- Joomla version -- : 3.8

Hi,
We set a rule that when cart total more than 500$ user can choose a gift into cart.
The problem is I put a module content module of gift in checkout page using module anywhere.
But when click on add to cart button, the cart module on the top right corner just don't response, but when I refresh page, the gift will show in cart.
Here is the url of the developing site:

diggo.com.tw
please login by test1/123456

and in checkout page you can see it.
I compare the add to cart URL from shop page and check out page, the only different is in Itemid, like
www.diggo.com.tw/index.php?option=com_hi...add=1&product_id=29& Itemid=632
www.diggo.com.tw/index.php?option=com_hi...add=1&product_id=29& Itemid=602
What is itemid? how can I change the one in checkout page?

Could you tell me why the add to cart button doesn't work in my checkout page? but it works fine in my other pages.
And what should I do to fix the problem or how can I change the itemid into other number?

Thanks in advance

Thanks, Nicolas,
I've fond the cause of the problem.
It was caused by a function I put in plugin as below.

	public function onAfterCartSave(&$element) {
		$ifCheckout = UJhtmlHelper::parseUrl($_SERVER['HTTP_REFERER'], 'ctrl');
		if($ifCheckout=='checkout') {
			$js =  '<script language="JavaScript">
	 				window.opener.location.reload();
	 				</script>';
			echo $js;
		}
	}