I'm getting a similar error message...
I'm using PHP & MySQL. eCart v4.5 with Authorize.net and my confirm.php to my checkout_success.php page transition is giving me the following...
Warning: require_once(WA_eCart/checkoutScripts/WA_simlib.php) [function.require-once]: failed to open stream: No such file or directory in /public_html/checkout_success.php on line 10
Fatal error: require_once() [function.require]: Failed opening required 'WA_eCart/checkoutScripts/WA_simlib.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /public_html/checkout_success.php on line 10
I have no /checkoutScripts/WA_simlib.php directory below WA_eCart.
Standard installation of the extension and a clean new cart creation from scratch...?