My friends sent a save the date card for their wedding...however,?

I won't be able to go. It's a postcard "save the date" and since the wedding is more than 6 months away, on the postcard it says they'll be sending a way to RSVP soon. I already know I can't go because of financial reasons (the wedding is across the country). I play no part in the wedding. Should I just email and tell her now I'll be unable to attend or wait for the official RSVP card to come in the mail?