code: 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;
      }
docs: Activates the usage charge.
  
  @return object