Yeah, I've been through a similar experience, didn't file RSU income correctly one year, they thought I owed $7k or something. I could have just paid it, but the amount was large enough and the issue simple enough to be worth arguing over. Wrote back with the correct adjusted cost basis to show I already paid the taxes when I sold them (I had done this correctly in other tax returns..) and ended up getting a check from the IRS for around $100 since there were some other minor ±couple dollar irregularities in a liquidated portfolio I inherited that were in my 'favor' with a capital loss. (Odd since I used that company's online tool, so the numbers should have been identical, but who knows.)
If you pay them, they go away. Or sometimes they pay you and go away. (My first IRS letter was after my first semester of college, which was a bit disturbing since my whole income that previous year was ~$1k, but opening it revealed that I had neglected some sort of new education credit and they were giving me a small check to account for it.) My only remaining "fear" of the IRS is the scary Audit, because I know my record keeping is terrible and I don't want the anxiety of calling places and hunting down receipts or getting sworn letters from people that such-and-such checks were used for rent sharing not income/paying for services, and so I only hope that any audit could just be resolved by paying a lump sum even if with a bunch of effort that could get reduced...
If you pay them, they go away. Or sometimes they pay you and go away. (My first IRS letter was after my first semester of college, which was a bit disturbing since my whole income that previous year was ~$1k, but opening it revealed that I had neglected some sort of new education credit and they were giving me a small check to account for it.) My only remaining "fear" of the IRS is the scary Audit, because I know my record keeping is terrible and I don't want the anxiety of calling places and hunting down receipts or getting sworn letters from people that such-and-such checks were used for rent sharing not income/paying for services, and so I only hope that any audit could just be resolved by paying a lump sum even if with a bunch of effort that could get reduced...