Problem with Callback page and Authorize.net

I’ve set up my Callback page. I’ve entered it in the Default Receipt Link URL in my Authorize.Net account. Whenever I submit a live charge and click the return link the Callback page reads:

Payment Failed
Sorry but there was an error during checkout.
No money has been taken from your account.

HOWEVER, the charges DO go through and I receive a receipt to our merchant email as well as a receipt to the submitter email.

Is there another step to setting up the Callback page?

Which of the methods should be selected on the Authorize.net page:
Link GET POST ?