From their website:
Redeeming Costco Cash Rewards
Cash back earned from your Costco Citi card will be provided as an annual credit card reward certificate once your February billing statement closes, and is redeemable for cash or merchandise at U.S. Costco Warehouses, including Puerto Rico.
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Can anyone clarify what the "redeemable for cash" means? If I bring in a reward certificate for $5000, they're going to give me cash out of the till?