The extension is for extra time to file your return. It doesn't extend the time to pay what you owe. They are due today, whether you extend or not. You can pay directly with the IRS here.
just file your returns today 4/15 by 11:59PM local time. the extension does not require that you file after 4/15
Form 2210 for the Underpayment Penalty? That's not for filing or paying late. And is not about last year. And not about paying estimates. The penalty is an estimated amount.
If you do not pay in enough tax from withholding and estimates, 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.
You might be able to eliminate it or at least reduce it. You can go to Federal Taxes tab or Personal tab, under Other Tax Situations and select Start by the Underpayment Penalties. You will answer a series of questions that may reduce or eliminate the penalty. Or you can elect to have the IRS figure the penalty for you. It's form 2210.
It's under
Federal or Personal (for Home & Business Desktop)
Other Tax Situations
Additional Tax Payments
Underpayment Penalties - Click the Start or update button