Credits are the site's shared balance. Each run shows its credit price before you submit: credits are reserved when you submit, returned if generation cannot be completed, and settled once it finishes. If a finished result cannot be delivered (for example it does not pass the safety check), its credits are returned automatically.
Each pack shows how long its credits stay valid, counted from successful payment; unused credits expire after that. Packs marked "never expires" do not expire.
When your balance is too low, the tool tells you; come back here to buy more.