tax owed to IRS

I owed $4,611 on my 2018 taxes. I was told a report would be sent the IRS and they woould get back to me by letter. That has not happened. I got a letter from the IRS yesterday telling me I have to pay the total due by Sept. 7 or I will have to pay a 1/2% monthly penalty as long as the payment is late. What is your responsibility in this matter? What do I do!!