Buy Store Credit (v15x)- Total Price become $0.00

Hi, i bought an extension called Buy Store Credit (v15x) <http://www.opencart.com/index.php?route ... e%20credit>. I am encountering some problem but the developer is not responding to my email and Skype message.

The Buy Store Credit is installed on my OpenCart Store, but it is not giving us the expected result. Any product purchased after installing this extension will result in Total: 0.00 in the Shopping Cart (screenshot: http://postimg.org/image/595lc5zd1). Thus, this messed up the store actual function.

I hope someone will help me to solve this. Let me know if u need more details. Thank you.