This seems fishy. You get notified in early January and they want payment by the end of January? Are you sure it's not a scam? I only say that because last year I got a letter saying that I under reported income and owed 10k. They thought we made extra money from selling stuff on Amazon, which we didn't. We've never sold anything on Amazon. It was clearly fraud and I wonder if that's what's going on? Or maybe a scam? That's what I thought mine was at first, but the letter was legit. Anyway, after sending a letter to the IRS explaining the situation, that didn't suffice. So we ended up filing a fraud claim with the FTC or some similar agency and notified the IRS accordingly. That did the trick and the IRS backed off. However, in all of that, they never demanded payment so soon. That gave us like 3 or 4 months to respond each time. But my cheeks did pucker so I definitely empathize. I consulted with a tax attorney at the beginning and he set me straight and just told me what forms to file. I can dig all that up if needed.