Question about how to respond to a threat of a levy when I have already lodged a dispute

I filed my 2021 taxes this year using the software on FreeTaxUSA. I was calculated to receive a refund. I mailed my return before 15 April 2025. This past September, I received a notice that the IRS changed my 1040 due to what they claimed was a miscalculation of the monthly contribution amount on Form 8962, Premium Tax Credit, which drastically reduced the PTC credit I was allowed, and that I owed a balance.

The notice gave no specifics about what was changed, so I took a couple hours to redo the calculations on my own. I determined that the only possible difference was the percentage of my AGI over the federal poverty line amount, which is used to help calculate the monthly contribution. In 2021 any taxpayer who received unemployed benefits would use 133% in these calculations. That is what FreeTaxUSA calculated. If the IRS computer did not take this into account, that would explain why they calculated my PTC credit differently.

I called the next day and between two phone calls, four agents, and getting disconnected twice over the course of six hours, I had no new information. I called the day after that and eventually reached an agent who heard my case and seemed to agree with me. He gave me his name and ID number and informed me it might take a few weeks for this information to change on the website.

Today I received a letter stating that they are threatening a levy if I don’t pay immediately. I received no response about that conversation I had with the agent about my dispute, and there is no record of that phone call on my account.

How should I respond next? Should I go through a formal appeal process, or should I send a letter directly to the IRS detailing the above info? Any advice is welcome.