@ croegner
ADDENDUM
You said you had a couple of quarters with unexpected income and that you increased your estimated tax payments for that, but that still could have left a quarter short. In addition to the info I left you above, if your 2020 income varied widely throughout the year, one can use the annualized income installment method to figure the penalty if it reduces or eliminates your tax penalty. That involves adding Schedule AI to the Form 2210. That's a lot of work for a $6 penalty, however, but it can be done in TurboTax, if desired.
I'm a fellow user, not a tax person, so any more info on that annualized installment method would have to come from someone else.
