noun
reminder letter; dunning letter; demand letter
A formal written notice requesting payment, a response, or the return of something. Often used in business or official contexts.
支払いが遅れているので、取引先に催促状を送った。
Because the payment was late, I sent a reminder letter to the client.
図書館
I received a reminder letter from the library for an overdue book.
請求書 is an invoice or bill requesting payment for goods or services, whereas 催促状 is a follow-up reminder after a payment or action is overdue.
催促 (reminder, urging) + 状 (letter, document). The compound follows a standard pattern for formal written notices.