Arizona 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.

Interest on any loan, indebtedness, or other obligation (other than medical debt) is 10% per annum "unless a different rate is contracted for in writing, in which event any rate of interest may be agreed to." Judgment interest on non-medical debt is the lesser of 10% per annum or 1% plus the prime rate (§ 44-1201(B)), unless a written agreement provides a rate. Verified on the official Arizona Legislature site (azleg.gov).

Arizona has effectively no usury cap for agreed rates: § 44-1201(A) expressly permits "any rate of interest" if contracted for in writing, including in B2B transactions (a 2022 amendment added caps for medical debt only). Agreed written finance charges on commercial invoices are therefore enforceable as to rate, subject only to general contract doctrines.

public+private-construction

Private construction: Arizona Prompt Payment Act (A.R.S. §§ 32-1129 to 32-1129.07) - owner deemed to approve billings 14 days after receipt, payment due 7 days after approval; late or wrongfully withheld amounts accrue interest at 1.5% per month. Public procurement/construction: A.R.S. §§ 41-2576, 41-2577 (and § 34-221 for public works) impose progress-payment deadlines with penalty interest at 1% per month. No general non-construction private-commercial prompt-pay act found.

Because any written agreed rate is lawful under § 44-1201(A), agreed interest-style late charges on B2B invoices face no usury ceiling; flat late fees remain subject to the ordinary liquidated-damages/penalty analysis (must be a reasonable forecast of damages). Absent any agreement, the 10% statutory rate applies to the overdue obligation.