That sounds fine. Use something comparable to what you're using for the other response cards and just include the email/phone as the method for those. I would be sure to include an RSVP By deadline, too.
One note: "Please RSVP" is pretty redundant because RSVP already has "s'il vous plait" in it which is French for please. I would probably say "Please respond to [email address] ...
Re: Wording for RSVP via email or phone