public function invoiceAmount($invNo)
    {
        $this->_checkInvNo($invNo);

        $digestSecret = self::digest(
            $this->colon($this->_eid, $invNo, $this->_secret)
        );

        $paramList = array(
            $this->_eid,
            $invNo,
            $digestSecret
        );

        self::printDebug('invoice_amount array', $paramList);

        $result = $this->xmlrpc_call('invoice_amount', $paramList);

        //Result is in cents, fix it.
        return ($result / 100);
    }