public function activate()
    {
        // Ensure we have a recurring charge
        $currentCharge = $this->shop->planCharge();
        if (!$currentCharge->isType(Charge::CHARGE_RECURRING)) {
            throw new Exception('Can only create usage charges for recurring charge.');
        }

        $this->response = $this->api->rest(
            'POST',
            "/admin/recurring_application_charges/{$currentCharge->charge_id}/usage_charges.json",
            [
                'usage_charge' => [
                    'price'       => $this->data['price'],
                    'description' => $this->data['description'],
                ],
            ]
        )->body->usage_charge;

        return $this->response;
    }