Is there a legal limit on late fees?

Limits vary by place and by the type of customer, and some places set none. Check the rules where you work before you state a fee on an invoice.

Where do I put the late fee?

In your terms, on the invoice, so the client sees it before it applies. The invoice tool has a Late fee line that prints your wording with the terms.

Enter what is overdue, the fee rate you state on your invoices and how many periods it has been late.

Enter the overdue balance to see the answer.

Late fee = overdue balance x rate per period x number of periods. The fee is not compounded: each period uses the original balance.

Whatever you charge should be written on the invoice or agreed before the work, not added afterwards.

Want these figures on an invoice? Make an invoice, free and with no signup.

Is there a legal limit on late fees?: Limits vary by place and by the type of customer, and some places set none. Check the rules where you work before you state a fee on an invoice.
Where do I put the late fee?: In your terms, on the invoice, so the client sees it before it applies. The invoice tool has a Late fee line that prints your wording with the terms.