One-time search credits: payment confirmation and technical-error reversals

We are evaluating Fincra as a possible checkout option for song finder, a browser-based music recognition service. It offers a one-time pack of 100 searches; completed searches consume credits, while technical errors should not. We have not integrated Fincra yet.

1. For a one-time credit-pack purchase, which verified Fincra event should trigger granting the search credits, and which payment identifier should make webhook retries idempotent?

2. If a recognition job fails technically after credits were reserved, is it best to restore those usage credits in our own ledger, separate from a payment refund?

3. If a customer requests a refund or a payment is reversed after some searches have been used, which events and transaction references should we rely on to reconcile the remaining credits?

Any Fincra documentation or sandbox examples for this type of one-time purchase would be helpful.