Fatal Error moving Universal Email receipt from confirm.php page to checkout_success.php page

I would like to move the Universal Email receipt to the checkout_success.php page. This way if there's a problem with sending the email it won't affect processing the order. However, when I move it to the checkout_success page and change the trigger to before page load I get a fatal error. It says no such file, but the file is there. Any advice? And if I don't change the trigger but leave it to what it was, will the email still send properly (I'm only changing it because that's the method I've used in the past and eCart 6 is still new to me).

Warning: require_once(webassist/forms/persist.php): failed to open stream: No such file or directory in /Applications/MAMP/htdocs/CustomStickersforGoodCauses/webassist/email/templates/wa_receipt.php on line 1

Fatal error: require_once(): Failed opening required 'webassist/forms/persist.php' (include_path='.:/Applications/MAMP/bin/php/php5.5.3/lib/php') in /Applications/MAMP/htdocs/CustomStickersforGoodCauses/webassist/email/templates/wa_receipt.php on line 1

Thanks!
~Melanie