Override group sidebar links

app/helpers/groups_helper.rb

 module GroupsHelper
+  prepend EE::GroupsHelper
+
   def group_nav_link_paths
     %w[groups#projects groups#edit ci_cd#show ldap_group_links#index hooks#index audit_events#index pipeline_quota#index]
   end

changelogs/unreleased-ee/fix-show-sidebar-sub-level-items-for-billing.yml

+---
+title: Override group sidebar links
+merge_request: 4234
+author: George Tsiolis
+type: fixed

ee/app/helpers/ee/groups_helper.rb

+module EE
+  module GroupsHelper
+    extend ::Gitlab::Utils::Override
+
+    override :group_nav_link_paths
+    def group_nav_link_paths
+      if ::Gitlab::CurrentSettings.should_check_namespace_plan? && can?(current_user, :admin_group, @group)
+        super + %w[billings#index]
+      else
+        super
+      end
+    end
+  end
+end