Once a sale is made an email with an invoice will automatically go out to the customers.
In a case where the customers did not receive it or request to email the invoice you can do that by following the below steps:
Be at the location you made the sale, go to the customer profile
In the customer's profile navigate to the payments page
Under order click on the order, whose invoice you want to email to the customer
When the order page opens, under the payments, click on email invoice
Under email invoice, you will be able to see the customer's email id, In case you want to send the invoice to another email, you can enter the email in this field and click on send invoice
An email with the invoice will be sent to the email id you have entered
