A customer may have received an product whilst at your business, to log this order on behalf of the customer, head to Store ➔ Orders ➔ Add an Order
You'll first be asked to select which customer this order is for:
Then, you can add the number of store items you would like to add to their order per variant:
Then once you click on "Place Order" you'll be redirected to the checkout screen to take payment.
For more info about the store, please see the following articles: