There's a big difference between a pre-authorization and an actual charge. Did they actually process a charge? A pre-auth shouldn't actually be charging you anything, and is just verifying the card is good and holding funds (which would still be a problem on a prepaid card). I've never actually checked to see if I get pre-auth requests on my card when I make a booking; it might be typical, but I've never noticed. However, incidentals should not have a pre-auth until you've actually checked in.