Here is a sample template you can use to write an order acknowledgment letter.
To [Recipient name]
Dear, [Recipient name]
This letter is to acknowledge your order for [Product/Service] placed on [Date]. Delivery of these [Products/Services] will be done within [Number] weeks. Your order number [Number] amounting to [Amount] will be delivered to the address [Address] as soon as possible.
We thank you for your business and hope to serve you in the future. The details of the products are attached along with the invoice for your order. Our company has an unmatched reputation and we hope to fulfill your needs to the best of our ability.
[Name of company]