To be honest, I think it'd be a waste of money to add programs at this point. You will be totally fine if you already got 2 invites. I applied to 42, which I think is plenty, but when all those programs I didn't apply to started making offers I was wishing I applied to more (I only applied to 8 of the 23 or so that have made offers). I'm impatient like everyone else and wish I was hearing from all the programs that I applied to. We still have a lot of time and a lot of programs are going to be making offers more than once. So, I'm sitting tight for now. The only programs I would feel comfortable adding are those that haven't downloaded applications yet. For me, only 1 of my 42 hasn't downloaded stuff yet, and that is Mt. Sinai. Maybe other people can post who hasn't downloaded apps yet and you can target those programs.
On the other hand, some programs might not review applications till November, but that means they won't be sending out invites until mid to late november. In that case, you won't be getting the reinforcement you're seeking right now and you'll have to wait another month anyway. By then, I bet you have more interviews and are feeling good, and you won't need those extra interviews from the programs that you weren't initially interested in.
Just some stuff to think about... but hold tight, and good luck. I'm sure that you'll be fine.