I guess I'm also not understanding. A virtual birthday party is not my cup of tea and I also don't regard it as a "real" party - so unless you told me YOU'D BETTER TELL ME IF YOU'RE NOT COMING (which would automatically get a response that I'm not coming and no explanation as to why because I don't owe you one), then I would just skip it.

If it were a real life party where food was being prepared or money invested, I definitely would RSVP. Again, no explanation if I'm not coming, because you just need to know yes/no in that case.

I'm thinking this is triggering something from your past because it is a very extreme reaction you are having to this imagined slight. She apologized. Either move on or you can end the friendship WITHOUT a long diatribe as to why.