I got married last year, and made some reasonable estimates of what my taxes would look like in 2012, the first full year of being married.
However, a few things have changed and some of the deductions I had last year, I don't get this year. So, I claimed one more exemption (maybe two) than I should have on my W-4.
As of mid-December, that's enough that my wife and I seem to have underwithheld our taxes by about $2500.
Unfortunately, we don't qualify for the safe harbor - we've paid less than 90% of what we owe (around $19000), and less than 100% of what we owed last year.
We're not self-employed, and don't have any meaningful income outside of our jobs. The deficiency is caused entirely by my extra exemption.
To avoid the penalty, can we still just send the IRS a 1040-ES and a check for about $2000 or so? The calculations behind it seem to be for small business people, but the actual form you send in just seems to be an SSN and a check.
This is in the United States.