Thanks DrByte for replying quickly.

I made the changes and tried another purchase, but it's still not going through. I mean, the transaction goes through, but the page is still blank and the transaction isn't recorded in the database, which in turn doesn't show up in the admin area.

However, the bank funds are withdrawn successfully.

I'm still getting this error from the Debugtool

Code:
[25-Sep-2009 07:36:11] PHP Fatal error:  Call to a member function update_credit_account() on a non-object in /home/.ashke/username/mystore.com/includes/classes/order.php on line 754
Any other advice? Thanks!