public function setCurrency($currency = 'USD')
    {
        $allowedCurrencies = ['AUD', 'BRL', 'CAD', 'CZK', 'DKK', 'EUR', 'HKD', 'HUF', 'ILS', 'INR', 'JPY', 'MYR', 'MXN', 'NOK', 'NZD', 'PHP', 'PLN', 'GBP', 'SGD', 'SEK', 'CHF', 'TWD', 'THB', 'USD', 'RUB'];

        // Check if provided currency is valid.
        if (!in_array($currency, $allowedCurrencies)) {
            throw new \Exception('Currency is not supported by PayPal.');
        }

        $this->currency = $currency;

        return $this;
    }