Where credits come from
Every verified paid invoice grants its allowance once per billing period. The grant is keyed to the invoice, so replaying the same webhook never doubles a balance. Opt-in welcome credits use an explicit start cutoff so old accounts are never retroactively gifted, and staff adjustments only ever land through the audited console with a reason attached.
Credits are granted, never simulated: a preview deployment shows no balance at all rather than a fake one.