So I just did my 2020 federal taxes with turbotax desktop and I owe the IRS $21.00.
A) Mail them a check for $21.00
B) Can the interest & penalties be figured out now? Then mail them a check for the $21.00 + Interest + Penalties.
C) ?
No. Just pay the Tax Due. The IRS will bill you for any interest or penalty. You can't figure it out.