E.J.
The non-rsvping mom needs to get over herself. You did nothing wrong.
It is a lot of time, planning and expense to have a party. That is rude.
If I were an attendee I would be so grateful to you for canceling the party.
Quick side note: my son was invited to a Bday party in preschool. We were not able to attend it that weekend(right before Halloween actually). I felt guilty until we go to school on Monday to find that class was canceled.....one of the kids had the H1N1 virus and shared it with ALL the other preschool kids that went to the party! Poor kids :-(.
So ya, you did the right thing!