Aha! a clue ...

Try this code instead so it checks against the total quantity of Products in the cart to ensure they are all products_id 1395 ...
Code:
if (!IS_ADMIN_FLAG) {
  // disable, (even when admin has it switched on):
  $this->enabled = false;
  if ($_SESSION['cart']->in_cart_check('products_id','1395') > 0 && $_SESSION['cart']->count_contents() == $_SESSION['cart']->in_cart_check('products_id','1395')) {
    $this->enabled = true;
  }
}