Kansas 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.

Creditors are allowed interest at 10% per annum when no other rate is agreed upon, for any money after it becomes due, money lent, money due on settlement of account from the day the balance is ascertained, money withheld by unreasonable and vexatious delay, and other money due where an express promise to pay interest was made. Verified on the official Kansas Revisor of Statutes site (ksrevisor.gov).

Yes. K.S.A. 16-207 caps contract interest at 15% per annum for notes/bonds/other written instruments, but the cap does NOT apply to 'business or agricultural' transactions - loans/credit sales made primarily for purposes other than personal, family or household purposes - so B2B credit is effectively exempt from the 15% usury cap. Consumer credit is governed separately by the Kansas UCCC.

public+private-construction

Kansas Fairness in Private Construction Contract Act: owners must pay undisputed contractor payment requests within 30 days; late undisputed amounts accrue interest at 18% per annum (contractors must pay subs within 7 business days of receipt, also at 18% if late), with a right to suspend work. A parallel Fairness in Public Construction Contract Act exists at K.S.A. 16-1901 et seq., and the Kansas Prompt Payment Act (K.S.A. 75-6403) governs state-agency payments for goods/services (rates under those two acts not independently verified here). No act covering ordinary private non-construction commercial invoices was found.

The private-construction act makes certain contract provisions (e.g., waivers of its payment protections) void as against public policy - agreed payment terms in construction contracts cannot waive the 18% late-interest right. Outside construction, agreed flat late fees on commercial invoices follow the general rule: enforceable if a reasonable liquidated-damages provision, not a penalty; the business-purpose usury exemption removes rate-cap concerns for agreed B2B finance charges.