The issue was with the discount amount... your original code had:
$totalDiscount += ($this->TotalColumn("TotalPrice") * ".$_SESSION['CouponDiscount'] .");//Result
The updated code had:
$totalDiscount += ($this->TotalColumn("TotalPrice") * $_SESSION['CouponDiscount']);//Result
It shouldn't have been based on the other code you removed... that code appears correct.
Does it all work now? Do you still need help?