Here is a sample template you can use to write a letter acknowledging a purchase order.
To, [Recipient name]
Dear, [Recipient name]
This letter is to confirm your purchase order number [Number] which we had received on [Date]. We are glad to accept your purchase and look forward to working with you in the future. As per the terms dictated in our quote, the delivery will take [Number] weeks from the date the order was placed. Payment should be made to the bank account details provided at the end of this letter.
If you have any queries concerning your order, feel free to contact us through our customer support number. Thank you again for your order. We will do our best to provide optimal service.