If necessary, a store administrator can write to a customer about their order from the order details page of the store's Admin area.
1. On the Orders list page (Orders -> Orders list), locate the order you require, and open its details for viewing/editing.
2. Switch to the Messages tab of the order details.
3. On the page that opens, type your message and click Submit.
This will start a communication thread on the order with a customer.
Now the customer will be able to read the message from their account and reply to you in the same thread.