After you file

The IRS is screwed up.

If you amend and pay your amended tax due now, IRS will send the money back to you because it takes so long to get to your 1040-X

This makes it impossible to pay by April 18th to avoid late payment.

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If you e-Filed 1040, you can e-File 1040-X and it will shoot right in there.

you have to pay any amount you owe on the IRS payments page.