[ThePowerOfFuet]

You're not wrong, but that is precisely the tradeoff: stored-value is more reliable but also more vulnerable.

It's reminiscent of the old NASA saying: "faster, better, cheaper: pick any two".

[account42]

You don't need the system to be invulnerable to fraud, you just need to be able to detect it. Offline stored value cards plus separately shipping transactions to a central system with eventual consistency can give you that. The vending machine in question probably isn't invulnerable to physical break in either.