Underpayment penalty

[CarolynM]

The US is a pay as you go system, so if you don't pay enough tax throughout the year, you face underpayment penalty.

The fee is normally waived if you paid 90% of your tax due, but it was lowered his year to not be applied if you paid 85% of the tax due.

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