When on the sales progression page you will need to create your invoice.
When the invoice is ready you can download it onto your PC
Then on the sales progression you can add the invoice to the private files section
Now you are able to attach it to the MOS email by clicking on edit and send on the MOS page
Then click on the attachment icon to browse the private files and attach the invoice
Don't forget to save!
Here is a short video that might also be helpful.