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Net-30 is a suggestion until you price the alternative.
A late fee clause turns "I'll pay you Friday" into a cost the client can see. This calculator computes the accrued fee from invoice amount, days past due, and your contracted monthly rate — plus the equivalent APR, so you can check the charge against your state's usury limits before it goes on an invoice.
Freelancers and agencies use it in the past-due reminder email — showing the client the exact accrued fee and total due gets checks written. Bookkeepers use it to apply contracted rates correctly across a whole aging report.
1–1.5% per month (12–18% APR) is the common commercial standard, but state usury laws cap it — ranging from about 6% to 18%+ APR depending on the state and whether the client is a business. The fee must be in the signed contract or terms accepted before work began; adding it after the fact rarely holds up.
Usually not the first time — the clause exists to change behavior, not to farm fees. Enforce it consistently for chronically late payers, waive it once for good clients with a reason, and always show the math (this calculator's total-due figure) in the reminder email. Visible arithmetic gets invoices paid faster than threats.
Discounts outperform fees for cash flow. A 2% discount for payment within 10 days ("2/10 net 30") costs you less than the average late fee you would collect and gets money in weeks earlier. Offer both: the discount as the carrot, the fee clause as the stick.