I am having a little problem with order update

-- url of the page with the problem -- : Private site (local server via virtual machine)
-- HikaShop version -- : 2.3.3 Starter
-- Joomla version -- : 3.3.6
-- PHP version -- : 5.3.14
-- Browser(s) name and version -- : Firefox 32.03

I'm creating a payment plugin for a local bank (FiBank). Haven't found one developed yet so, im having the honor of bashing my head in the wall reading *bank* documentation(dont ask why,please). Anyway my problem is that when i do :
$orderClass = hikashop_get('class.order');
$order = $orderClass->loadFullOrder($order_id, true, false); // im passing the current order_id
$order->cart->shipping_address->test= 'blalba; // test is a custom field that i have created thru hikashopp .. starter...
$orderClass->save($order);
And on the page where it should re-direct me to the payment gateway, I've disabled the auto-redirect, since i need to see if im correctly assigning the "fields" in the form that's going to be sent to the bank. So there i get this error.

"#1054 Unknown column 'order_subtotal' in 'field list' SQL=UPDATE `dsk2o_hikashop_order` SET `order_billing_address_id`='2',`order_shipping_address_id`='2',`order_user_id`='1',`order_status`='created',`order_type`='sale',`order_number`='D8Q3',`order_created`='1413380390',`order_modified`='1413380390',`order_invoice_id`='0',`order_invoice_number`='',`order_invoice_created`='0',`order_currency_id`='30',`order_full_price`='40',`order_tax_info`='a:1:{s:0:\"\";O:8:\"stdClass\":3:{s:11:\"tax_namekey\";s:0:\"\";s:8:\"tax_rate\";s:7:\"0.00000\";s:10:\"tax_amount\";d:0;}}',`order_discount_code`='',`order_discount_price`='0.00000',`order_discount_tax`='0.00000',`order_payment_id`='8',`order_payment_method`='fiBank',`order_payment_price`='0.00000',`order_payment_params`='',`order_shipping_id`='2',`order_shipping_method`='manual',`order_shipping_price`='5.00000',`order_shipping_tax`='0.00000',`order_shipping_params`='O:8:\"stdClass\":1:{s:6:\"prices\";a:1:{s:3:\"2@0\";O:8:\"stdClass\":2:{s:14:\"price_with_tax\";d:5;s:3:\"tax\";d:0;}}}',`order_partner_id`='0',`order_partner_price`='0.00000',`order_partner_paid`='0',`order_partner_currency_id`='0',`order_ip`='192.168.79.1',`order_site_id`='',`order_subtotal`='35',`order_subtotal_no_vat`='35' WHERE `order_id`='83' "

I am doing that in - function onAfterOrderConfirm(&$order,&$methods,$method_id) .
So i am wondering is there a way to edit specific part of the order and update it ?
Or what could be the cause for my problem, if you could enlighten me ?
Order_subtotal - the whole order class is supplied by you .. shouldn't it be able to save it ?

Shipping address is individual for each order isn't it ? What i mean is, if i modify that test field and save it via address class, it will save it only for the current order right ?

Hi,

The change will be done for each orders made, so each new used shipping addresses.