Hi qouiou ,
I've checked the code of my collegue and your website and found out the cart automatically closes if you click outside that element.
If you click the Buttonizer button, it indeed opens the cart, but the cart closes as it registers a click outside the cart container.
What you could do is adding an setTimeout.
Can you try using the following code:
const el = document.querySelector("#cart");
setTimeout(function() {
if(el.classList.contains("open")) el.classList.remove("open"); else el.classList.add("open");
}, 500);
Let me know if it helps 🙂
