We did not send out formal invites. They were just unnecessary and I didn't want to pay for them.
I sent a postcard STD 1 year in advance. About a month later I sent an email "invitation" with resort details and the form to book their rooms and RSVP. (And my Travel Agent followed up with another email after mine)
I don't regret the decision at all, although I do hope my guests aren't like "where the hell is my formal invite?!?!" What sucks is my SST says "formal invite to follow" because I bought them and mailed them before I decided against actual formal invites. Ooops.