Error Message on Checking out

Please help,

I’m getting this error when you hit the “Proceed to Secure Checkout Button”

Fatal error: Call to a member function checkCartInventory() on a non-object in /home/content/96/10021196/html/wp-content/plugins/simple-e-commerce-shopping-cart/models/SimpleEcommCart.php on line 524

My website is: http://www.realestateshotz.com

Any Help would be greatly appreciated!
Frank

http://ww.wp.xz.cn/extend/plugins/simple-e-commerce-shopping-cart/