Because under normal circumstances, this kind of problem should not appear.

Or you can try not to book tickets on the website, call the local ticketing phone and ask him to deduct money from your credit card on the phone. See if it's possible. Domestic customers can tell him the card number directly on the phone, and then he can deduct the money directly. I'm not sure about Singapore. Why didn't you buy a domestic round-trip ticket? It's convenient and cheap.