Filing by mail and paying online?

[zha0q1]

I am filing my federal tax by mail this year. Can I still pay online with https://www.irs.gov/payments/direct-pay?

Then, what do I attach as evidence of payment to the printed forms? If I have underpaid, would IRS let me know and make up for the difference?

[VolvoGirl]

Yes you can pay Online.  You don't put anything on your tax return.  The payment will match up to your tax return.  How would you underpay?  You pay the tax due on 1040 line 37.  If the IRS adjusts your return they will mail you a letter and bill.  

[zha0q1]

Great thanks! Do I mail to a different tax center if it's paid online?

[VolvoGirl]

Where to mail federal paper returns

https://www.irs.gov/uac/where-to-file-paper-tax-returns-with-or-without-a-payment