Engagement Invites/STD Cards

I am planning on making my own engagement invites. I never sent out STD cards. Do you think it is necessary to do so? Maybe I could combine the engagement invite with a STD????? Could I put all the info on one? Or maybe I could do two seperates and mail them in one envelope. Or do you think I could just skip the STD altogether??? Thanks!

This is what we are doing. We are having an engagement party and for those that are attending, their favors are the frame place cards and a magnet saying Wanda and Carl; Thank you for coming!; Save the date!; March 5, 2004.

For those that are not attending the eparty we are sending them an std saying Wanda and Carl; Psst… Save the date March 5, 2004. I will include LI travel info and hotel info with a letter.