The important thing is to not over-think things; it's rarely that case that you truly, honestly, really need real-time automated fraud detection.
Start with implementing the absolute bare minimum. You'll then receive emails from e.g. Sift when a bad user is identified, and you can manually refund the transaction, cancel the order, and block the user.
I'm thinking about integrating with siftscience. How trustworthy are they? It looks like you need to send them information about your users and give them script access on your page in order to help identify fraudulent transactions. (which is completely reasonable but still requires a lot of trust on our part)
I really like https://siftscience.com.
The important thing is to not over-think things; it's rarely that case that you truly, honestly, really need real-time automated fraud detection.
Start with implementing the absolute bare minimum. You'll then receive emails from e.g. Sift when a bad user is identified, and you can manually refund the transaction, cancel the order, and block the user.