No, there is no additional cost to you. However, if you choose FERS, you may elect to make additional TSP contributions (known as make-up contributions). These are contributions that you could have made if you had been correctly covered by FERS. Of course, you're the one who chooses how much additional contributions you want to make.
There was an unexpected error when performing your action.
Your error has been logged and the appropriate people notified. You may close this message and try your command again, perhaps after refreshing the page. If you continue to experience issues, please notify the site administrator.