hopefully, you retained that certified mail receipt. send a copy (not the original) of it along with a letter to the address in the notice explaining that, under penalties of perjury, enclosed is a copy of the certified mail receipt showing that the extension was mailed timely on 3/14/2020. Therefore, you request abatement of the penalties (and interest - omit unless there are interest charges - there shouldn't be any) because there was a timely filed extension. if you didn't retain it, you'll have to ask the IRS to abate the penalties because this was the first time they were asserted.
