In your code you are only creating one custom field with your add_post_meta() usage. Then when you use the get_post_meta() there is only one value in the post. Can you go into the actual post and look at the custom fields. Is there more than one custom field with the same “extras” key? If not, try an iteration to create multiple custom fields for your post:
$extras = $_POST['extras'];
for ($i = 0, $l = count($_POST['extras']); $i < $l; $i++)
{
add_post_meta($pid, 'extras', $_POST['extras'][$i]), true);
}