David,
The Deltapay module DOES work with the osC's checkout_process.php ( $Id: checkout_process.php,v 1.128 2003/05/28 18:00:29 hpdl Exp $).
The reason that it does not work with CRE6.2 is that the following code is never called by 6.2's process.
Code:
$payment_modules->before_process();
Instead, this code is lost in a series of if statements pertaining to various supported payment gateways(around line 64); and because Deltapay is not built-in, this crucial function, the very function that posts/gets the error codes and displays the bright red error messages is always bypassed and the user is sent to checkout_success rather than back to checkout_payment to try again.
I hard coded the line into CRE6.2 and all is well (unless I broke something else by adding the line.)
I won't post this as a bug in the tracker because it may be peculiar to the Deltapay payment module, but I think that the developer responsible for maintaining the checkout_process.php code should be made aware of the problem, because it may break other payment gateway contributions, too.
Hard coding must be seen as a very temporary work around and not a permanent fix because it makes future patch copying dangerous.
If you care to see the screen shots of the working payment error messages in CRE6.2, let me know and I'll post them.
Keep up the good work. I'm making money with CRE and I hope you are, too.