The confirm script needs to be implemented on the order confirmation page that is only reachable after the payment was made.
Add this script tag to the body of your HTML:

      <script type="text/javascript">
        var ShoeSizeShoppingCartConfirmation = {
          shopID: "SHOP_ID",
          orderID: ORDER_ID

        // Do not modify
        (function() {
          var script = document.createElement('script');
          script.type = 'text/javascript';
          script.src = ''+
            '&sid='+Math.round(new Date().getTime()/1000);
          (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(script);
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