Impossible to pass callback for payment module.

Hi
I almost finished payment module. It redirects customer to gateway and returns him to success page after payment.
Processing of success with /index.php?Checkout&Process module passes normally, but it also makes direct background server2server callback.


Situation:
I tried to use /index.php?Checkout&Callback url and callback() method in module for this purpose.


Actual behavior:
Checkout Controller makes same verifications as for usual customer, so server fails to pass them.
Server fails to be authenticated user and to have an existing shopping cart.


Expected behavior:

Remove human verifications for callback and let request to pass to callback() handler of a module.

osCommerce version:
3.0.2

Best regards,
Vladimir