code: public function index(Plan $plan)
      {
          // If the plan is null, get a plan
          if (is_null($plan) || ($plan && !$plan->exists)) {
              $plan = Plan::where('on_install', true)->first();
          }
  
          // Get the confirmation URL
          $bp = new BillingPlan(ShopifyApp::shop(), $plan);
          $url = $bp->confirmationUrl();
  
          // Do a fullpage redirect
          return View::make('shopify-app::billing.fullpage_redirect', compact('url'));
      }
docs: Redirects to billing screen for Shopify.
  
  @param \OhMyBrew\ShopifyApp\Models\Plan $plan The plan.
  
  @return \Illuminate\View\View