Mike,
The problem mentioned by chirag doesn’t come if I use an upgraded wordpress but a different issue arises.
I changed get_permalink for the pay page with $order->get_checkout_payment_url(). In new woocommerce >2.1 I have set my callback URL to checkout page that had a successful transaction by payzippy but returning page shows:
Checkout
Sorry, this order is invalid and cannot be paid for.
Edit
