You can do this, and should be able to fairly simply. I'm not a Zen Cart expert so this is the best I can do:
\includes\modules\pages\checkout_process\header_php.php
That's the file you'll want to edit. Change the line:
Code:
zen_redirect(zen_href_link(FILENAME_CHECKOUT_SUCCESS, '', 'SSL'));
to redirect where you need to instead. The FILENAME_CHECKOUT_SUCCESS is defined in: \includes\filenames.php
So there you can add your own definition like define('FILENAME_CUSTOM_CHECKOUT_SUCCESS', 'checkout_custom_success');
then make a checkout_custom_success.php file that has the contents you want. Then in that page you can redirect it to the normal checkout success page.
Hope that helps..
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