The fraud analysis is AI driven and, not perfect. Treat it as a red flag and do your own analysis of the situation to determine if the purchase throws up any other red flags.
If the customer left a phone number, you can call him/her and just say you're doing an order verification, and verify whatever information you feel is necessary and pay attention to the person's disposition when answering.
In the end, it's your decision whether to fulfill the order or issue a refund. Weigh out how suspicious the order seems, the dollar amount of the order, and whether the risk of a potential chargeback or the risk of losing an otherwise potential repeat customer is greater to you.
If you decide to refund, just email or message the customer and let them know that you are unfortunately unable to fulfill their order at this time due to an issue with the payment method on the order.
as @BobsMojo has mentioned, Shopify's basic fraud analysis is quite basic and can throw out false positives since it just serves as a precaution for merchants. Your best-recommended course of action is to just contact the customer over the phone or email them to make sure they have made the purchase.
If it's a relatively high-value ticket item, then I would make sure to ask the customer to confirm their home address.
Furthermore, I recommend you read up on an article I wrote regarding going deeper into investigating the customer's order https://medium.com/@jason_80572/five-things-you-can-look-at-to-prevent-fraud-orders-from-happening-t...
I hope this helps you a bit.