I see Costco is offering $500 certificates on Alaska Air for $449.99. Looks like a good deal since there is no expiration and you just deposit them into My Wallet. They're good on any fare class, but "Certificates can not be used towards premium upgrades." Anybody know what this means? I'm not sure if it means you can't pay the upfare difference to move into premium class or if it would prevent you from applying a Gold Guest Upgrade when you purchased a qualifying fare with U space available. Any insight would be appreciated.