You can send Invoices, Delivery Notes and Return forms directly to your customers if they have provided an email address during the order process.
First go into the Orders table and press on the order you want to manage (I have chosen the latest one for this example).
Once inside the Order, look to the right to see customer order details.
Right at the bottom of the Customer area you will find an Email button.
Pressing on it will bring out a drop-down with multiple selections like Invoice, Returns Form and Delivery Note.
To send out an email to your customer (to the email address right above the button). Just click on either of the options and a small confirmation will appear.
As you've noticed the initially grey button icon is now orange, this indicates that some of the documents have been sent, but not all of them. We can confirm that by clicking on the drop-down again.
The invoice document has changed to green to indicate that it has already been sent. And if we sent the returns form and the delivery note, the drop down button would change to green as well.
If you want to re-send the Invoice, simply click on it again.
You can also send messages en-masse directly from the Order window.
Simply select the orders who's Invoices you'd like to send to the customer.
Press on the grey Email button at the top row and select the documents that you want to send. Press on the selection and thats it, documents are now sent.
This feature is not available for Amazon and ManoMano integrations, as those Sales Channels forbid you from messaging the customers directly though their email.