Need to add permanent checkout checkbox

-- HikaShop version -- : 2.3.1
-- Joomla version -- : 3.3.1
-- PHP version -- : 5.4.43
-- Browser(s) name and version -- : MOZILLA FF 33.0.3

Hello there Hika experts:
I need to add a checkbox to my checkout page where client can choose to add extra payment for special package handling and delivery ,the extra service always costs the same regardless the package.

I thought of adding a payment method where I always add the extra sum ,but I have two payment methods so this make my checkout page have 4 payment methods which entirely not practical:
example
- Pay with Tranzila .
- Pay with Tranzila with extra handling (+20$) .
- Pay with PayPal ...................................................... <=== This I can do but is not elegant !!
- Pay with PayPal with extra handling (+20$) .

So how do I go about this ,in an elegant way preferably ?

Thanks for your reply,

I have added a custom field to the address table with the name express that can either have the value yes or no (single drop down list).
Now I want to create a plugin that goes something like this:

class plgHikashopExpress extends JPlugin {
	function onAfterCartShippingLoad(&$cart) {
		$app = JFactory::getApplication();
		$order_fields = $app->getUserState(HIKASHOP_COMPONENT.'.checkout_fields', null);
		
		if((int)$address_fields->express == 'no')
			return;

		if(!empty($cart->order_currency_id))
			$currency_id = (int)$cart->order_currency_id;
		else
			$currency_id = (int)hikashop_getCurrency();

		$express = new stdClass();
		$express->name = 'Express Delivery';
		$express->value = '';
		$express->price_currency_id = $currency_id;
		$express->price_value = 20;
		$express->price_value_with_tax = 20;
		$express->additional['express'] = $express; 
	}
}

I have managed to install the plugin ,but I now know that the code I tried won't work.
I am trying now to access the express field in the address object in my cart ,to see if express delivery is tecked or not so I can add extra 20 to the payment or not.

Sorry for the hassle.
It all worked out just fine:
I added extra field to address named it 'express'
then I installed the plugin based on the function onAfterCartShippingLoad(&$cart)
then I got the user id:
$hk_user_id = $app->getUserState(HIKASHOP_COMPONENT.'.user_id');
then I got the express state:
$db = JFactory::getDbO();
$db->setQuery('SELECT express FROM #__hikashop_address WHERE address_user_id =' . $hk_user_id);
$express_state = $db->loadObject();
And now it is all working the I want it to.

Thanks a lot for the help.