Once the file has been generated in Unix, it is immediately ready to be uploaded.
The SFTP credentials are :
ServerName: sftp.birchstreet.net
Username: Nhaserot
Password: NjdkYzUyNzY5
Port: 22
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I have been uploading the file to the folder: /home/Nhaserot/invoice and that seems to have done the trick (did this two days in a row without anyone complaining).