Pennsylvania late fee law

6%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.

6% per annum: any reference to the 'legal rate of interest,' and any documented obligation to pay 'with interest' without a specified rate, is construed as 6% per annum (Loan Interest and Protection Law, Act 6 of 1974). Exact text verified via FindLaw's reproduction and corroborated by the official PA General Assembly act listing (official site was unreachable for direct fetch).

Parties may contract for a higher rate; Act 6's 6% usury ceiling (41 P.S. § 201) applies principally to loans of $50,000 or less that are not otherwise exempt - business loans and corporate borrowers are generally outside the cap and may agree to market rates. Not verified against official § 201 text; verify exemption details with counsel.

public+private-construction

CASPA (73 P.S. § 501 et seq.) covers private non-residential construction (and residential over 6 units): owners must pay contractors per contract terms, contractors must pay subcontractors within 14 days of receiving payment; wrongful nonpayment carries interest of 1% per month plus a penalty of 1% per month plus attorney fees. Public contracts: Commonwealth Procurement Code prompt-payment provisions (62 Pa.C.S. § 3931 et seq.) require timely progress payments with interest at the rate the Secretary of Revenue sets for overdue taxes. No general private commercial (non-construction) prompt-payment statute.

CASPA's interest/penalty provisions cannot be effectively waived in most respects (2018 amendments strengthened anti-waiver protections). Outside construction, flat late fees on commercial invoices follow ordinary liquidated-damages/penalty doctrine - enforceable if agreed and reasonable. Verify with counsel.