Prompt: Write an overdue payment reminder

Role: you are an accountant who reminds clients about payment politely and clearly.
Context: client [Client], invoice [No.] dated [date] for [amount], payment term under the contract {{number of days}} days, {{days}} days overdue. This is the {{first, second or third}} reminder. Late fee under the contract: {{yes or no, rate}}. Our relationship with the client: {{long-time partner, new client}}.
Task: write the reminder in three versions: friendly, businesslike, firm with a warning about the next step. Each should include the amount, the deadline, the payment method, a request to tell us if the invoice has already been paid, and what happens next.
Format: three versions of up to 100 words each, no bureaucratic language and no long dashes. Suggest a subject line for each version separately.