My dad had a similar experience with the IRS. He had startd a small business and was supposed to be paying something monthly, but he thought it was quarterly. He got fined, and when he mailed in his payment, he included a letter explaining that it was his first time running a business, and it was an honest mistake. ~6 months later he got a check back from the IRS for the fine amount, plus interest. They seem to be pretty lenient as long as you aren't actively trying to make their lives difficult.