Hey!
I can understand your frustration, but as a parent of two boys that have been invited to MANY parties this year all I can say is if you don't hear from us, we're not coming. Sometimes the boys were given invitations that didn't even have their name on it (so I'm assuming the parent sent in invitations for everyone and the teacher was to hand them out). I have a thing about hating to call people, especially if I don't even know you (I just get nervous about it). If there was an email address on there, I'd be first in line to let you know my son wouldn't be making it.
I did call and RSVP one time. The parent had actually sent in a reminder for the kids to RSVP and because of that, I called. She actually seemed surprised when I told her he wouldn't be coming. It was like she was expecting RSVP's only from those that would be attending