The documentation should be made available online as soon as you sign up with them - how else you gonna use their system. By the same token, I would be surprised if they gave any information out to anyone who was not an authorised service user.
Be aware that they all state that 'it is the internet and a callback cannot be guaranteed. The only definative record of transactions is the one available to you through your secured user admin area' or words to that effect. And it is true. Use callbacks to say thanks or sorry, and default to some inconclusive message if there is none, but don't rely on a confirmation callback to trigger the despatch of goods, you could upset people if their payment is processed and you miss it. And loose money if you don't confirm the payment by other means. To easy to spoof the callback to your site and all these payment processors come under 'denial of service' attacks at regular intervals
