I’m not sure if this is something we’re going to be able to support due to the way it works.
If you upload a document manually through the document manager, you can assign it to a folder directly, and it’s just that - it’s a document, not necessarily a receipt or invoice.
With Dropbox and the email feature however, this is tied to the receipt hub. The receipt hub treats everything coming in as an invoice or receipt and displays a preview of the files for you to type in the details. As far as I’m aware (but I will double check), this isn’t something that’s possible with a zip file.