I just started to use Simple eCommerce for tuition payments on one of my websites and after activating it everything seemed fine, but I notice that my forms from gravity forms were not showing the confirmation page correctly. I contacted them for support and we were able to determine that I was getting this error when the confirmation page was displayed.
[16-Jul-2012 21:40:22 UTC] PHP Fatal error: Class 'SimpleEcommCartGravityReader' not found in /home/clong/public_html/wp-content/plugins/simple-e-commerce-shopping-cart/models/SimpleEcommCartShortcodeManager.php on line 447
If I disable the Simple eCommerce error goes away and the forms work properly. I do not get any errors from gravity forms through out my testing. Other than Simple eCommerce blocking the confirm page Gravity forms works fine.