You haven't told us what was on your Turbo Tax return. Was it a refund on line 35a or a tax due on 37? How much? Were you expecting the $4042 refund you got on Nov 9 or is that what the letter was for?
Did your extension payment show up on the right line? For 2021 Federal Extension payments should be on schedule 3 line 10. And go to 1040 line 31. If it showed up on line 26 as an estimated payment the IRS missed it and added it in. But then they will eventually see you didn't make the Estimated payment on line 26 and ask for it back.
