Quarterly Estimates are not like Payroll tax payments. They are against that person's entire 1040. If they have a working spouse, that person could increase their W2 withholding so that the spouse with the Sched C wouldn't even have to pay quarterly estimates. Or, the business just doesn't contribute much to the entire 1040. That's why it's not a business Payment. It was part of 1040 payments all along.