Your only real option, if you're not willing to call, is to write back to the IRS on the notice. Explain your concerns, and your suspicion that the $400 might be a penalty of some sort.
It's my opinion that it's not a penalty, as there would normally be a computation in the letter that would reflect a penalty, and the IRS would generally cite the Internal Revenue Code when assessing a penalty.
Looking at a transcript as suggested by ErnieS0 would also be a good start, just to see if you can locate a $400 difference anywhere. You can identify the codes on the transcript by looking in Section 8A, Master File Codes - Transaction, MF and IDRS Collection Status, Freeze and IDRS Status 48, Restrictive and Filing Requirements in the 2022 version of Document 6209 - ADP and IDRS Information,
I know you're not keen on calling the IRS, but I will point out that it has a callback system that can use to connect you with a representative without waiting on hold. If you'll call the IRS at (800) 829-1040, you should be able to follow prompts to find the callback option.
