authorize.net successful payment redirect. Relay Response page? Or a better way?

I have giftcards I'm selling on my wordpress site. I am sending buyers to Authorize.net Simple Checkout.

Now I want to be able to track sales in GA, so I want to automatically redirect them to a page on my site. I set up a Relay Response URL on authorize.net, but it's giving me an Error:
This transaction cannot be accepted.

I'm pretty sure that means it tried to record a response and could'nt because this needs to be coded to accept responses from Authorize.net.

Maybe someone can give a hint on the proper setup of WordPress/Authorize.net/Relay Responces or is there a better way to automatically redirect successful purchases back to the site?