Hope you are well.
Please be advised that you can set up different conditions based on which a certain template will be picked up for an order. Since you have set up a separate invoice template for your returns, you could simply add the following condition for that invoice template which will instruct the system to only pick it for order returns.
The above status is how a return is specified within a linnworks database. More information on how to set up a PDF attachment can be found here: http://www.linnworks.com/Doc/PDF_Invoice_Email_Attachment
Info on printing conditions is available here: https://www.linnworks.com/support/settings/template-designer/using-template-designer/printing-conditions
I wonder how can I use a different invoice template to include in my return notification email. I am trying to send a pdf document with my return notification email and it should be different than the one I send with dispatch notification email.