Expressions for Business Letters

Invoice
Please find enclosed invoice no. … for …
Thank you for your order.
We look forward to doing business with you again.
Reminder
According to our records, we have not yet received a remittance for above invoice.
May we remind you that your payment is overdue for three months.
This is to remind you that above invoice is still unpaid.
We would appreciate if you cleared your account within the next days.
Please send your payment promptly.
If you have already sent your payment, please disregard this letter.