you might want to look very carefully at the invitation, as you are correct that it's rather sketchy to invite someone on two days notice. is it a reputable conference? did they add you at the last minute because they felt they needed to check some box they'd failed at originally? be judicious.

I appreciate your warnings, Prof. Cramer! It is a reputable conference, and one that I was planning to attend anyways. I don't know why they're adding me last-minute (I suspect some lack of planning or forethought), but the symposium is related to my work, so I'm inclined to accept, annoyance aside.