Update combobox fail - php

Question

i'm trying to use comboboxes for quantity in Virtuemart instead of quantity text and update button.
When i have only one article in the cart it works perfectly, but when i have more than one, it's not working.
Here's the form code
<form action="<?php echo JRoute::_ ('index.php'); ?>" method="post" class="inline" name="frm">
<input type="hidden" name="option" value="com_virtuemart"/>
<input type="text" title="<?php echo JText::_ ('COM_VIRTUEMART_CART_UPDATE') ?>.2" class="inputbox" size="3" maxlength="4" name="quantity" value="<?php echo $prow->quantity ?>" style="display:none;"/>
<select name="cantidad" id="cantidad" onchange="getval(this);" value="<?php echo $prow->quantity ?>" title="<?php echo JText::_ ('COM_VIRTUEMART_CART_UPDATE') ?>"> <script type="text/javascript">
function getval(sel) {
document.frm.quantity.value = (sel.value);
document.getElementById('actualizar').click();
}
</script>
<option value="<?php echo $prow->quantity ?>"><?php echo $prow->quantity ?></option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>
<option value="7">7</option>
<option value="8">8</option>
<option value="9">9</option>
<option value="10">10</option>
<option value="11">11</option>
<option value="12">12</option>
<option value="13">13</option>
<option value="14">14</option>
<option value="15">15</option>
<option value="16">16</option>
<option value="17">17</option>
<option value="18">18</option>
<option value="19">19</option>
<option value="20">20</option>
</select>
<input type="hidden" name="view" value="cart"/>
<input type="hidden" name="task" value="update"/>
<input type="hidden" name="cart_virtuemart_product_id" value="<?php echo $prow->cart_item_id ?>"/>
<input type="submit" class="vmicon vm2-add_quantity_cart" id="actualizar" name="update" title="<?php echo JText::_ ('COM_VIRTUEMART_CART_UPDATE') ?>" align="middle" value=" " style="display:none;"/>
</form>
Thank you