Was there an "insert" in that letter explaining that the due date is July 15 and the computers couldn't be stopped from sending out those notices (or something to that effect)? See https://proconnect.intuit.com/community/tax-talk/discussion/irs-bills-look-at-the-insert-not-the-not...
Checks sent in April apparently haven't been cashed. I just heard from another client. He sent by certified mail. Everything is piled up in mail trailers.
The letter the clients received did have an insert stating the payment due date was July 15, 2020. What a waste of paper mailing out these notices stating taxes are due June 22nd and then including the insert stating the payment is due July 15th without penalties.