“Self-employed people only pay tax once a year, when they file.”

What the law says

Federal income tax is pay-as-you-go. Employees pay through withholding. If you have income with no withholding on it, estimated payments are the substitute.
The underpayment penalty can apply even if you pay everything by the filing deadline, because it is about paying enough as the year goes.

Source: IRC §6654 · Pub 505 · Instructions for Form 1040. Read the full rule, with the law quoted

Not advice. Not a filing. Verify with a licensed professional. Rules change by year and facts.

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