So for 2020 I filed my tax returns. However there was a discrepancy that I needed to correct so I amended it. Got that sorted out thinking it was over. Eventually got a letter saying I didn’t owe them anything. Thinking the matter was resolved, I threw that away. Today, got a letter from the IRS in the mail dated as June 28th saying I owed them $16.00 by July 28th, which has obviously passed. This letter doesn’t seem to mention the amendment I made so I wondered if it was an old notice from before. If it is then nothing to worry about. But if it isn’t, wtf happened?