Late payment fees and interest: what's enforceable
Adding a late fee to your invoice is easy. Making it stick — and actually getting paid — has rules most freelancers never learn.
A late fee line on an invoice is a deterrent more than a revenue stream. Used well, it shortens days-to-payment and makes your terms feel real. Used badly, it sours the relationship and sometimes isn't even enforceable. The difference comes down to a few small details that most templates skip.
You can only charge what you agreed to
Slapping a fee on an overdue invoice you never warned the client about is, in most jurisdictions, not enforceable. The late-fee terms have to be in the contract, the SOW, or — at minimum — on the original invoice itself, agreed to before the work started. Adding it after the fact is just an angry email with a number on it.
Reasonable fee structures
- A flat fee per late invoice (e.g. $25 after 14 days overdue).
- A monthly percentage of the outstanding amount (commonly 1–1.5%).
- Statutory interest, where local law provides one (see below).
- A combination: small flat fee plus monthly interest accrual.
Whatever you pick, write it in plain language: "Invoices unpaid after 30 days accrue interest at 1.5% per month on the outstanding balance." Specific is enforceable. Vague is not.
Statutory interest (the underused tool)
Many jurisdictions give businesses an automatic right to charge interest on overdue commercial invoices, even without a contract clause. In the UK, the Late Payment of Commercial Debts Act sets the rate at 8% above the Bank of England base rate plus a fixed recovery fee. The EU has a similar directive. The US leaves it to state law, with a usury cap that varies. Look up the rule for your jurisdiction once — you'll likely find you have more leverage than you thought.
The escalation ladder
A late fee works best as one rung on a clear escalation ladder. Day 1 past due: a friendly nudge. Day 7: a firm reminder that late fees begin accruing on day 14. Day 14: an invoice update showing the new total with the fee added. Day 30: a final notice mentioning collections or small-claims court as the next step. Each rung gives the client a chance to pay before the next one is unpleasant.
When to waive the fee
Good clients sometimes pay late for boring reasons — a holiday, an out-sick AP person, a bank delay. If the client apologises, pays in full, and doesn't have a history of this, waive the fee and say so explicitly: "I've removed the late fee on this one." The goodwill is worth more than the $25. Repeat offenders get the fee, every time, no exceptions.
The deterrent effect
Most clients with a published late fee on every invoice pay faster than clients without one — not because the fee is huge but because the invoice signals that you track these things. That signal alone is worth adding the line, even on invoices you'd never actually enforce it on.
What to put on the invoice itself
A single line near the payment terms: "Payment due within 14 days. Overdue balances accrue interest at 1.5% per month." Keep it factual, not threatening. AetherInvoice templates include this as a configurable footer field so the language stays consistent across every invoice you send.