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prueba lo siguiente

/httpdocs/themes/xxxxx/assets/js/custom.js 

añade este codigo

$(document).ready(function () {
  prestashop.blockcart = prestashop.blockcart || {};

  var showModal = prestashop.blockcart.showModal || function (modal) {
    var $body = $('body');
    $body.append(modal);
    $body.one('click', '#blockcart-modal', function (event) {
      if (event.target.id === 'blockcart-modal') {
        $(event.target).remove();
      }
    });
  };

  $(document).ready(function () {
    prestashop.on(
      'updateCart',
      function (event) {
        var refreshURL = $('.blockcart').data('refresh-url');
        var requestData = {};

        if (event && event.reason) {
          requestData = {
            id_product_attribute: event.reason.idProductAttribute,
            id_product: event.reason.idProduct,
            action: event.reason.linkAction
          };
        }

        $.post(refreshURL, requestData).then(function (resp) {
          $('.blockcart').replaceWith($(resp.preview).find('.blockcart'));
          if (resp.modal) {
            showModal(resp.modal);
          }
        }).fail(function (resp) {
          prestashop.emit('handleError', {eventType: 'updateShoppingCart', resp: resp});
        });
      }
    );
  });
});

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