Handling image-upscaling credits and partial batch failures with Fincra

We are evaluating Fincra for upscal image, a browser-based image upscaling service with paid processing credits and batch jobs. We have not integrated Fincra yet and want to plan the payment flow correctly.

1. For credit-pack purchases, should credits be granted only after a verified payment webhook? Which event or transaction ID is safest to use as an idempotency key?

2. If a batch contains several images and only some jobs succeed, should we handle processing-credit returns entirely in our own ledger, separately from payment refunds?

3. If a subscription renewal payment later fails or is reversed, what is the recommended way to reconcile unused credits without affecting completed image jobs?

Pointers to the relevant Fincra events and sandbox testing approach would be helpful.