How to mitigate a large underpayment penalty for estimated taxes resulting from an amended return submitted in Aug; made 3 payments btwn 8-30 & 1-10, still expect refund?

If you amended your 2023 return in August, 2024, and submitted additional payment at that time, there would be interest and late payment penalty charged, as well as underpayment penalty if applicable.

If you feel you have a basis for abatement of any outstanding penalties assessed, you can file Form 843. For more details, see the Form 843 Instructions.

The IRS may reduce an underpayment penalty if any of the following apply:

• You or your spouse (if you file a joint return) retired in the past 2 years after reaching age 62 or became disabled and you had reasonable cause to underpay or pay your estimated tax late. See Waiver of Penalty in Instructions for Form 2210PDF.
• You had most of your income tax withheld early in the year instead of spreading it equally through the year. 
• Your income varies during the year. Complete Form 2210, Schedule AI, Annualized Income Installment Method PDF(found within the form).

The penalty for underpayment of estimated tax can also be removed to the extent that the underpayment is the result of a casualty, local disaster, or other unusual circumstance such that it would not be fair to impose the penalty.

See this IRS webpage for more information about conditions under which IRS may abate a penalty.