Another option is to use the Save As Droplet feature in Transmit.
This creates a small app that will accept files and upload to that server, using Transmit behind the scenes.
So then you can just use Retrobatch’s Open In App.
I just tested this method with my Dropbox and it worked a treat!
(If you end up adding multiple Droplets then look into Transmit’s DockSend feature to make your life easier)