How to trigger add $10 from radio selection?

How to trigger add $10 from radio selection?

I have a custom field "radio" in my check out page. i want to be able to add $10 to this if it's not null or false. would the simplest way be to just add some PHP code to a file somewhere to add $10 to the shopping cart?

i have most figured out but what code would i need to just add $10 extra if radio box was true on checkout page?

Why then use of the if else? can you explain the code more if you can? i just want to understand for future resources.. thanks..

i now have the radio selection in my field step of checkout do i add this code there instead?

the Radio selection field is now in the fields step of check out, i will be putting the custom fields differently using the code you gave me on the other post about echoing each field separately.

so this code

$already = false;  
foreach($this->rows as $i => $row){  
 if($row->product_code =='10_dollars_product') $already = true;
}  
if($this->order->custom_field_column_name){
 if(!$already){
  $app =& JFactory::getApplication();  
  $app->redirect(hikashop::completeLink('product&task=updatecart&product_id=106&quantity=1&add=0&return='.urlencode(base64_encode(hikashop::currentUrl()))));  
 }
}else{
 if($already){
  $app =& JFactory::getApplication();  
  $app->redirect(hikashop::completeLink('product&task=updatecart&product_id=106&quantity=0&add=0&return='.urlencode(base64_encode(hikashop::currentUrl()))));  
 }
}