New Hampshire late fee law

10%per year

Attaches to the debt by operation of law when you agreed no rate, so it accrues before any lawsuit and can be stated on a demand letter.

Two-part statute, verified on the official General Court site. RSA 336:1, I: the annual rate of interest 'in all business transactions in which interest is paid or secured, unless otherwise agreed upon in writing, shall equal 10 percent' - this 10% is the default when a B2B contract is silent. RSA 336:1, II is the formula-based judgment/prejudgment rate: the prevailing discount rate on 26-week U.S. Treasury bills at the last auction preceding the last day of September, plus 2 percentage points, rounded to the nearest tenth, determined annually by the state treasurer and effective each Jan 1–Dec 31.

The 10% default applies only 'unless otherwise agreed upon in writing,' so parties may contract for a different rate. New Hampshire has no general usury ceiling applicable to commercial/B2B transactions (specialty consumer-lending statutes aside) - but this absence-of-cap point was not verified against a specific statute this session; verify with counsel.

none-found

New Hampshire is repeatedly identified as the only state with no prompt-payment statute even for public construction projects, and it has no private-construction or general commercial prompt-payment act. Payment timing and late-payment interest are governed by contract terms, common law, and the UCC; contractors rely on mechanic's liens (RSA 447) and breach-of-contract remedies for leverage.

No statute specific to flat late fees on commercial invoices was found; agreed late fees are enforceable subject to the liquidated-damages/penalty doctrine. Note that absent any agreement, the RSA 336:1, I default of 10% interest is available on overdue business obligations.