Shopping cart / Payment module / checkout process

[zizzor.com]

I have been working on a payment module and have it %99 working.

When I look at other payment modules, they pass "sucess url" to the transaction server(actionurl). After the transaction is successful, the transaction server redirects to the sucess url without any parameters.

On other payment modules, all the transaction server is doing, is redirecting to checkout_process.php. Nothing seems to be passed to checkout_process.php. checkout_process.php automatically redirects to checkout_confirmation.php, which automatically redirects to checkout_success.php. At this point the shopping cart is empty and the transaction is recorded.

My payment module successfully makes the payment to the transaction server. When it redirects back to checkout_process.php, I get a credit card processing error.

Somehow, checkout process isn't being told the transaction was successful.

Does anybody know where I should start troubleshooting this?