Late payment: interest is a contract term, not a mood
You can charge interest or late fees only if the contract or the applicable law provides for them. Otherwise the only lever is escalation, not a surcharge.
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Check the contract first
Look for payment terms, late payment interest and any flat fee.
If the contract is silent, the default rules of the jurisdiction may still allow statutory interest — that is a question for a lawyer, not for a template.
How to state it
Factually: «Per clause 4.2, interest of X% applies from the due date.»
Once, not in every email. Repeating it reads as bargaining, not invoicing.
When to waive it
When the delay was caused by your own invoice error — wrong PO number, wrong bank details.
When waiving it gets the principal paid today. Interest is a lever, not revenue.
What actually recovers money
A named human on the phone with authority to agree a plan.
A written payment plan with two dates.
Escalation you actually carry out.
Frequently asked
Can I add a late fee after the fact?
Only if the contract allows it. Adding a fee to an invoice that did not mention it usually produces a dispute instead of a payment.
Should I stop work on an overdue client?
That is a commercial decision, and it is often the most effective reminder you have. Decide it deliberately, not in anger.
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DueBuddy — invoices and reminders, without the enterprise contract