When I said "its not that hard" I meant:

1. Save the metadata hash that dropbox's API provides you and use that hash for further requests.
   
2. When you edit a local file save in an inventory which file it was.
   
3. Ask dropbox's API for modified files using the metadata hash.
   
4. If HTTP 304, skip to 8
   
5. Compare the response with files modified locally, if there is a conflict ask the user what to do.
   
6. Upload or Download conflicting files.
   
7. Download new remote files.
   
8. Upload new local files.
   
9. Clear your modified files inventory and ask dropbox for a new metadata hash.
   
10. Repeat
    

With deep directories warn the user that it will have an impact in terms of performance, and encourage her to sync specific folders instead of her root directory.

Just do what other applications like Nebulous Lite does, its not that hard, if a free app offers it, why not a paid one like yours?

If App Store had a refund policy like Google Play I would have already asked for a refund.