I'm curious if you ever got an answer on this. When I try and use that AWD, the website shows "your member discount has been applied" as well as my savings by using it (I assume versus rack rates), but it does NOT say what B270898 actually makes me a member of

The actual COSTCO travel website shows AWD A108326 for AVIS rentals. No mention of B270898 there. Internet searches seem to *think* its tied to COSTCO but I can't find any actual current references if that's still the case.
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B270898 has some pretty hefty savings, including on one-way rentals in markets where I have a hard time getting cheap options.