Uploading files to a shared folder

If you only need to upload the files to a single folder via the API and don't need to otherwise share the folder with others via Dropbox, the best thing to do here would be to register an app for the "app folder" permission. Access tokens for apps with this permission can only access the special "app folder" created for the app in the connected account. This can't be set to a pre-existing folder though, nor can the special app folder be made into a shared folder, or contain or be contained in a shared folder.

Otherwise, the Dropbox API unfortunately doesn't offer a way to register an app or access token for access to a specific existing folder only unfortunately.

You could use a shared folder and connect a full Dropbox app to your own account though, as you described, but if an app folder setup would suffice, that would be safer.