Step 1 To Generate an Invoice
Navigate to 'Order Info' >> 'Order'
right click the icon 'Transfer into Invoice' on the right panel and generate an invoice

invoice1

Step 2 To Send the Invoice to Customers
To see the invoice, navigate to 'Order Info' >> 'Invoice'
Right-click the icon 'Email' and confirm

* * Other things you can do to the invoice * *
View: right click the icon 'View' to see the invoice
Edit: right click the icon 'Edit' to edit the invoice
Download: right click the icon 'Download' to download the invoice
Payment: right click the icon 'Payment' to add payment history

invoice2