How can do a completely silent install? More than a /S please.

While an end-user is logged in and working, I deploy Dropbox 13.4.21 Offline Installer.exe with the /S switch, via a third-party software deployment agent, and the Dropbox deployment works almost perfectly.  Except that my end-users still get the final Dropbox Setup screen.  This is the screen that prompts for Email and Password.  They then call me asking about this screen that came out of nowhere, and I have to say "Close that window and worry about it later when you need to use Dropbox, if ever."

If they manually close this window via the X, I get where I need to be:  Dropbox installed on the PC, icons where appropriate in the Start Menu, and Dropbox running in the tray, in a "Waiting to be linked to a Dropbox account..." state.

Is there a way to do a completely silent install that does not prompt for credentials at the end.  Also, is there a comprehensive list of all command-line installation switches?  I know about /S and /D.  Wondering if there are others.......

Thank you