This is what we’re done before too, if you’re staying at a self catering place you’ll need food/snacks/drinks etc so the extra person’s money covered that and the first round of drinks when we went out.
I agree! I think they think they’ve found a free online RSVP service but haven’t realised it’s asking everyone for bank details to sign up! I’d message them separately and let them know the problem, bet they’re super embarrassed