The current interest rate on the late payments is about 3% APR, so don’t let that frighten you into acting in a rush and making a mistake or overpaying.
Usually, the IRS will send a letter that proposes a tax amount that you owe. If you agree with this amount, reply to the letter and send a check. However, the tax amount is sometimes incorrect, especially in a case like this where the IRS does not know your cost basis. You should prepare an amended 2018 tax return, add the sale of stock and all the associated information, and calculate the amount of tax you owe. If it is the same as the IRS bill, just mail a check in the IRS will adjust your tax return in their file. If you have calculated a different amount, then mail a check for the new amount, plus a copy of your amended tax return and a letter of explanation, to the address of the office that sent you the notification.
Don’t be afraid of the interest, but do pay attention to any deadlines in the notification letter. If you miss those deadlines, then the proposed tax amount becomes final and you can’t appeal it by performing the calculation.
