I underpaid my 2021 estimated taxes but paid more than 110% of last year's taxes, so as I understand it I will not be liable for a corresponding underpayment penalty. I just discovered the under payment but given the date (after Jan. 18) I'm not allowed to make an additional estimated payment online for 2021. Should I send the payment in by mail even though I'm not likely to be assessed an underpayment penalty or am I missing something here?
