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According to my friend who was a SVP at Costco, this is, in fact, a primary reason. It's to mark the receipt so it cannot be used again.

• brk 8y ago
  Can’t the cashier do that?
  • simplicio 8y ago
    The idea is to mark when you're leaving the store, so you need someone actually standing by the exit. If the cashier did it, it wouldn't really prevent the scam. You could still circle back into the store and skip the cashier on the second run.