I’ve overpaid my tax, they told me about it and IIRC I just got one letter with a check in it, I didn’t need to do anything. I’ve also underpaid and my experie

I’ve overpaid my tax, they told me about it and IIRC I just got one letter with a check in it, I didn’t need to do anything. I’ve also underpaid and my experience was basically the same (except I had to send a check instead obviously). Edit: and the “penalty” was just modest interest on the underpayment.