Extension of time to file DOES NOT extend the time you have to pay tax in full.
April 15 (or the first Federal workday after that) is the due date for being paid in full to duck penalties.
I was charged for failing to pay on time AND for filing late. The extension is supposed to give taxpayers extra time to file without penalties.
Don’t confuse penalties for late filing with penalties for not paying on time. Different penalties, neither of which is particularly bad. And it’s my experience that late payment starts January 1, not April 15.