I see, the the cart object file, instead of referring to the cart object by name, it should be referred to as this.
i changed line 162:
$totalCharge += floatVal("".(($dmbc_in_stock->TotalColumn('TotalPrice') - ($dmbc_in_stock->GetDiscounts()) *1) * $_SESSION['TaxRate']) ."");//Result
to:
$totalCharge += floatVal("".(($this->TotalColumn('TotalPrice') - ($this->GetDiscounts()) *1) * $_SESSION['TaxRate']) ."");//Result