So how much do I owe in taxes?

Some thoughts here:

* A lot of people's taxes changed for 2020 due to COVID and various federal legislation passed in response.

* Outstanding IRS bills, in general, are a common fraud vector (although admittedly not via US Postal Mail).

* Ignored notices eventually start accruing interest/penalties (although the ordering seems to refute this theory).

As with any interaction with any level of government, your first step should always be to contact your elected representative to get their constituent services staffer looking into it on your behalf.