Right, you made estimated payments, and it was exactly the right way. You paid based on a reasonable estimation.
The point is it's pretty hard to just avoid paying taxes all year (either by plain not paying your self-employment taxes or misrepresenting your situation on a W-4), bank it, and come out clean.
The point is it's pretty hard to just avoid paying taxes all year (either by plain not paying your self-employment taxes or misrepresenting your situation on a W-4), bank it, and come out clean.