What's the initial payment size? If it's really low then you are being used by card testers. If so, increase the initial payment amount. Your goal here should

What's the initial payment size? If it's really low then you are being used by card testers. If so, increase the initial payment amount. Your goal here should be to make the friction on the card tester/fraudster enough that they move on to someone else while not derailing your actual prospective customers. Obviously, block all temp email providers. Maybe temporarily add an extra step for @gmail subscribers, like a phone confirmation. Introduce delays that wouldn't bother a normal customer but would bother a card tester.