Плюс\минус в корзине VirtueMart 2 - [VM2&3] Общие вопросы и ошибки

Здравствуйте! Подскажите, пожалуйста, как исправить этот код, чтобы в корзине VirtueMart 2 можно было увеличивать или уменьшать количество товаров?

      $step=$prow->min_order_level;
            if ($prow->step_order_level)
               $step=$prow->step_order_level;
            else
               $step=1;
            if($step==0)
               $step=1;
            $alert=JText::sprintf ('COM_VIRTUEMART_WRONG_AMOUNT_ADDED', $step);

            ?>
<form action="<?php echo JRoute::_ ('index.php'); ?>" method="post" class="inline">
<input name="quantity" type="hidden" value="<?php echo $step ?>" />
            <script type="text/javascript">
      function check(obj) {
       // use the modulus operator '%' to see if there is a remainder
      remainder=obj.value % <?php echo $step?>;
      quantity=obj.value;
       if (remainder  != 0) {
          alert('<?php echo $alert?>!');
          obj.value = quantity-remainder;
         
          return false;
          }
         
       return true;
       }
</script>
      
         <input type="hidden" name="option" value="com_virtuemart"/>
         
               <!-- <label for="quantity<?php echo $this->product->virtuemart_product_id; ?>" class="quantity_box"><?php echo JText::_ ('COM_VIRTUEMART_CART_QUANTITY'); ?>: </label> -->
               <span class="quantity-box">
                  <input type="text" class="quantity-input js-recalculate" name="quantity[]" onclick="check(this);"
                        value="<?php if (isset($this->product->step_order_level) && (int)$this->product->step_order_level > 0) {
                           echo $this->product->step_order_level;
                        } else if(!empty($this->product->min_order_level)){
                           echo $this->product->min_order_level;
                        }else {
                           echo '1';
                        } ?>"/>
               </span>
               
               <span class="quantity-controls js-recalculate">
               <input type="button" class="quantity-controls quantity-plus"/>
               <input type="button" class="quantity-controls quantity-minus"/>
               </span>
           
         
      </form>