Why was there an estimated tax penalty even I filed the return before 4/15? How could I get that back?

[CarolynM]

Because the US is a pay as you go system, you can face underpayment penalty, even when receiving a refund.

You can not pay the fee and respond to the IRS when they seek it, but you are unlikely to succeed.

For more information, follow this link: IRS on Underpayment Penalties