Thank you Kelvyn, that was a great tip - I found the absolute path.
However - it is still not working as intended. I set up a folder "VM" in the root of my site. Within the directory, I created the folder "invoices":
I then created the path /homepages/11/a12345678/htdocs/mysitefoldername/VM/ in "Shop/Templates/Path for Sale"
I am then uploading a PDF-file, which I want to sell, in "Shop/Media Files/New".
The file exists in the folder (proved with Filezilla), but I am recognizing this problem:
/homepages/11/a12345678/htdocs/mysitefoldername/VM//homepages/11/a12345678/htdocs/mysitefoldername/VM/ :: Unwriteable
--> by adding the full path, then the server is considering it twice obviously?
When I then change the status from PENDING to CONFIRMED, an invoice is created and sent out the customer. Fine.
--> here the next problem - there is no download link included for the PDF? The customer is not getting the file...