public function _requestPayload()
    {
        $payload = [
            'authorization_code'    => $this->authorization,
            'amount'                => $this->amount,
            'reference'             => $this->transactionRef,
            'email'                 => $this->email,
        ];

        if (!empty($this->plan)) {
            $payload['plan'] = $this->plan;
        }

        return $this->toJson($payload);
    }