[onyx]

The bottom line is thus: CostCo is not about to get stuck with a bad cheque or an invalid credit card purchase in the stated large sums. The transactions, however fraudulent, had to be valid in payment. No CostCo employee or manager could possibly allow a multi-thousand dollar purchase to leave the store without first confirming either the validity of the cheque or the credit card.

[spycatcher]

That's why a manager might approve a large cash sale rather than have customers walk out the door. Probably just thought they owned a candy store. Don't forget the terrorists used valid credit cards too. Who knows? The FBI is keeping info close to the vest.