You seem to have a plugin that changes the DOM structure.
If you inspect your site, you’ll find that the whole page content is wrapped inside a <div id="ocs-site"...>
The default DOM structure for this is: Demo Site
<body>
<nav> </nav> <--this is where you normally place the z-index so the cart items goes on top of the sticky add to cart.
.
<-- page contents Example: div#page -->
.
<div id="wc_sticky_cart_panel"></div> <-- this is the sticky add to cary panel.
</body>
As you can see, it’s working well because the DOM structure isn’t altered.
But your site instead has this structure:
<body>
<div id="ocs-site">
<nav> </nav>
<-- page contents Example: div#page -->
</div>
<div id="wc_sticky_cart_panel"></div> <-- this is the sticky add to cary panel.
</body>
Removing the plugin that causes this <div id="ocs-site">
wrap on page should address the issue.