Was it for an underpayment penalty on 1040 line 38? Did your return have a penalty listed? It is just an estimated amount. It's common for the IRS to send you a bill for more or even a refund if you have an amount on line 38. It's from form 2210.
If you do not pay enough tax through withholding, you may have to pay a penalty for underpayment of estimated tax. Even if you are getting a refund you can still owe a penalty for not paying in evenly during the year. Generally, most taxpayers will avoid this penalty if they owe less than $1,000 in tax after subtracting their withholdings and credits, or if they paid at least 90% of the tax for the current year, or 100% of the tax shown on the return for the prior year, whichever is smaller. It is included in your tax due or reduces your refund.
