We charge the cost of the appointment immediately after the it occurs, never before. If a charge fails, the system will automatically retry the charge again up to four times. So if the charge immediately after the appointment fails, the system will automatically retry after 1 day. If that fails, it will try again 2 days after that first retry. If that retry still fails, it will wait 3 additional days before trying again. And finally it does a fourth and final retry 4 days after that.
If every attempt to bill a card fails, an email is sent saying that Fons will stop trying to charge the card and to contact Fons directly to have us look into the issue.