public function toOptionArray() {
        return [
            [
                'value' => self::CARD_VISA,
                'label' => __('Visa')
            ],
            [
                'value' => self::CARD_MASTERCARD,
                'label' => __('Mastercard')
            ],
            [
                'value' => self::CARD_AMEX,
                'label' => __('American Express')
            ],
            [
                'value' => self::CARD_JCB,
                'label' => __('JCB')
            ],
            [
                'value' => self::CARD_DISCOVER,
                'label' => __('Discover')
            ],
        ];
    }