public function setPayment(string $provider_token, string $currency = 'EUR')
    {
        if (!$provider_token) {
            $this->logger->warning('setPayment expects a valid provider token, an invalid one given.');
            throw new BotException('Invalid provider token given to "setPayment"');
        }

        $this->_provider_token = $provider_token;
        $this->_payment_currency = $currency;
    }