Work out a fair late fee, then generate a ready-to-send reminder message matched to how overdue the invoice actually is.
Matched automatically to how overdue this invoice is, friendly for a few days late, firmer the longer it goes unpaid.
A late fee only holds up if it was disclosed upfront, ideally in writing on the original invoice or contract, before the work started. Adding one retroactively to an invoice that never mentioned it is a much harder conversation, and not one most clients will accept gracefully.
For the full staged approach to following up on a late invoice without damaging the relationship, see the chasing late invoices guide.