A plain page with the two-line snippet pasted in. Add a few things to the cart to see the flow. Payments on this demo shop are switched off; on yours, the button opens Stripe.
The whole page is this: <script src="https://addtheshop.com/shop.js" data-store="pk_ats_86b27a2a36842041cb3d78c60092eb6a63c4" async></script>
<div id="addtheshop"></div>