You might want to call the phone number on the letter just to make sure you still need to submit, considering you received your refund.
If you can’t get a live person to help you, though, you should go ahead and send in the documentation, and within the time period specified in the letter. You don’t want to be told later that you didn’t provide the information requested.
Generally you won’t be contacted about a return after a refund based on that return is issued, but it does happen. Based on what you’re saying, it’s not an audit, they just need additional information, for whatever reason.
