I recently received a notice stating that I owed a penalty and interest for unpaid 2019 taxes. I suspect that I am not the only one and this is what I think of a the Law of Unintended Consequences. Typically, a guesstimate my taxes and file by April 15. For 2019, I finished and sent a check for the entire amount due before the July 15, 2020 extended deadline. Bear in mind that if I had filed for extension on April 15, 2020 and paid by check that I would have had till October 15, 2020 for the IRS to have processed my payment. But because the filing date was extended to July 15, 2020 the payment by check was NOT considered a payment made by July 15, 2020 so a penalty and interest was added. So if you get a similar notice it could be because you did not pay all of the taxes due by July 15, 2020. Moral of the story, pay online.
