code: public function process(Plan $plan)
      {
          // Activate the plan and save
          $shop = ShopifyApp::shop();
          $bp = new BillingPlan($shop, $plan);
          $bp->setChargeId(Request::query('charge_id'));
          $bp->activate();
          $bp->save();
  
          // All good, update the shop's plan and take them off freemium (if applicable)
          $shop->update([
              'freemium' => false,
              'plan_id'  => $plan->id,
          ]);
  
          // Go to homepage of app
          return Redirect::route('home')->with('success', 'billing');
      }
docs: Processes the response from the customer.
  
  @param \OhMyBrew\ShopifyApp\Models\Plan $plan The plan.
  
  @return \Illuminate\Http\RedirectResponse