Acknowledge letter: An acknowledgement letter is a document used by businesses to formally acknowledge the receipt of something, for example, an invoice, that can be used as a courtesy, for record keeping, or to raise an issue with whatever service was received.
Announce letter: A letter of announcement is a letter that announces a special event or occasion that people need to be aware of. Letters of announcement are usually informal, and state in clear, concise terms what the event is and what further action should be taken by the recipient.