You can only charge a late fee your terms stated before the work was done, and many US states cap the interest rate. Most B2B businesses include late-fee language in their terms and rarely apply it; its value is as a lever at the escalation step, not as revenue.
Glossary
Late fee
Also: late payment penalty · finance charge
A late fee is a charge added to an invoice that is paid after its due date, usually a flat amount or a monthly percentage of the balance.
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