You could adjust the line that is adding the extra price to account for this like so:
$ATC_itemPrice = floatval("2");// column binding for the price from the add to cart
if(isset($_POST['size']) && strpos($_POST['size'], 'xxl') !== false) $ATC_itemPrice += floatval("1.50");
I just added a strpos check for the xxl value within the posted element, if the xxl string is present in the posted value then it will add 1.50 to the price.