Do not just send them a
payment. Contact the IRS first, and you may want to elicit the help of a local CPA.
The notice you received should have the exact reason why it was sent to you. I mean, the IRS is usually very clear as to why they think that you owe them money. In this case, ask them what in particular about estimated tax payments are they inquiring about. Are they saying that you should have paid more? Clearly the answer here is that they think you do, but what I'm saying is talk to them and find out why.
Now, there is an IRS website that explains why you got the exact notice that you received. It is helpful to review this website to find out how to respond to the notice they sent (but I still believe its a good idea to contact a live agent). The website to review is:
https://www.irs.gov/individuals/understanding-your-irs-notice-or-letter
Once you know exactly why the IRS is saying that you owe them more money, go back to the tax return in question and determine if your original tax return is correct or incorrect...not according to the notice, but rather correct or incorrect as to common interpretation of tax laws/guidelines. If you disagree with the IRS's decision, you must communicate that to them. Be prepared to defend your position with documents and consistent, sound reasoning. You are not obligated to acquiesce to their decision. This is why I suggest a local CPA.
However, if you did inadvertently forget to include any income, there may
be little you can do except pay the tax that would have been due with the original return to begin with. In this case, if you can show that you did not leave the
income off in an attempt to skirt your responsibilities, then you should be
able to get the IRS to abate any penalties. However, they will not forgive any
interest due in a situation where you did not include all your income. Again, this is why I suggest a local CPA.
