For unpaid invoices you can create a friendly reminder or a formal dunning letter — and email it when needed.
Open Invoices → Unpaid and tap the invoice you need.
In invoice detail, open the menu at the top right (three dots).
Choose Create reminder, set type and date, then save. To send: open the menu again → Send.
Tip: Add a late fee as a line on the customer’s next invoice first, finish the invoice, then create the dunning letter.