Invoicing can be customized with via Invoicing Parameters.
First highlight the booking on the booking grid, then access the 'Print' option from the right click menu
OR from the top menu bar
Click on the name of the template you would like to use and wait for the merge to complete
Once the document has been merged and an invoice number assigned, you will see a 'Document has downloaded' message.
Check your downloads folder for the merged document and alter formatting as needed.
To attach the document to the booking for later reference, click NEXT and follow the onscreen instructions (screenshots below).
To attach later, click CLOSE. You can attach any document to the booking at any time from the 'View Attachments' window
Once the documents have been uploaded click the green checkmark 
to exit.