Physical wedding invitations are still tradition such that people may get offended if they do not get one in the mail.
Anyone so offended by the lack of a physical invitation that they don't come to my wedding is someone I wouldn't want at my wedding anyway. Two birds, one stone
Anyway, it's not totally relevant to the main story here, since that was very obviously a "This is the official invitation/confirmation" thing, but I think it's worth considering that if you are going to buck tradition, you should try to be absolutely clear that that's what you're doing so that people can adjust.