IANAL but breach of contract. You have made an offer through your website, the customer has accepted that order through your website, at which point by default

IANAL but breach of contract. You have made an offer through your website, the customer has accepted that order through your website, at which point by default a contract exists between you. You can wiggle round it a bit with your T+Cs, but if you are giving them every impression that a contract exists which you can fulfil then they are allowed to rely on a contract existing. If you've made no attempt to charge their card, that's your fault, not something you can blame on the customer. If there were too many false positives I would also expect to attract attention from Trading Standards --- false advertising? --- and they might be satisfied that you're making honest mistakes, or they might insist that you overhaul your fraud detection procedures or your customer communications, but either way they will suck up some of your time. (This is from a UK perspective.)