are what I thought of first, too. Is there a version that's used by more than one company yet?
A big difference between that kind of money and Amazon's would be how "free" the money is for the administrators to distribute. When it's used by the game administrators to, for example, compensate a player for something that went wrong, it has zero cost for the administrators to use that way. There's only the opportunity cost of what that player might have bought otherwise, but it's considered part of not losing a customer so not exactly.
When real products, not virtual products, and lower tier vendors are involved, it can no longer be thought of by the administrators as having zero cost to distribute, so it becomes more like a currency than a voucher, but still a bit voucher-like.
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