Day 1-7 overdue: assume it's a mistake
Most late payments are forgotten, not refused. Send a short, friendly reminder with the invoice attached, the amount, the due date and a direct payment link. Remove every step between reading the email and paying.
Day 8-30: firm reminder and the late fee
If your contract has a late payment clause, now is the time to cite it: "Per section 4, a 1.5% monthly late fee applies from [date]." The late fee calculator gives you the exact amount and a ready-to-send email. If your contract has no such clause, you can still ask, but you cannot enforce it, which is why it belongs in every contract.
Day 30+: pause work and put it in writing
Stop delivering new work until the balance is paid, and say so in writing. If ownership of your work only transfers on full payment (it should), remind the client that they do not yet hold the rights to use it. Then send a formal demand letter with a final deadline, usually 10-14 days.
Last resort: small claims or a collection agency
Small claims court is designed for amounts like most freelance invoices, and a signed contract with a clear scope is the most important piece of evidence you can bring. Collection agencies typically keep 25-50% of what they recover, so use them only when the alternative is zero.
Prevention beats collection
A deposit at signing means you are never owed the full amount. Draft it now with Clinch: describe the job in a sentence, get a clean draft, send one link. Your client signs from their phone without an account, and the deposit is paid in the same step.