Three letters with escalating pressure: polite → firm → final
Subject: Invoice #[N] — reminder\n\nHi! Reminding that the [date] payment deadline for invoice #[N] for [amount] has passed. Probably just buried — duplicating payment details: [details]. Please confirm when to expect payment.
Subject: Invoice #[N] — 10 days overdue\n\nThis is the second reminder. Per contract [clause] the payment date was [date]; now 10 days overdue, amount [amount] + penalty [X]/day per terms.\n\nPlease pay by [deadline] or give a concrete date. Next — work suspension and formal claim procedure.
Subject: Invoice #[N] — final notice\n\nDespite reminders on [date] and [date], the debt of [amount] remains unpaid. Last voluntary payment deadline — [date].\n\nNext: formal claim with contract and acts attached, collection costs at your expense [contract clause]. Public debtor registries — also a consequence.\n\nWork with you is suspended until settlement.
Receivables discipline: reminder on day+3, not day+30. Each letter — short, with amount and dates. Penalties and suspension only work if they're in the contract — add 'penalty X%/day' and 'right to suspend after >N days overdue' clauses.