Sorry, can't help you there. I don't know precisely what a flexible spending account is, or why/how they would audit it. However, it sounds like some kind of debit card to me. In any case, it will be treated like a Visa credit card at most vendors. Even debit cards are put through as credit most of the time. So, assuming the card is in good standing and has available balance to cover the appointment there should be no problems. Just present it or treat it like any other Visa.