They allow one extra driver for free on rental cars. Most people just say F that and their spouse/SO drives even if they don't out them down on the rental agreement, but my wife is in insurance so everything has to be by the book. Quite the gestapo PITA about it sometimes too.
As far as savings, I always check there and then check expedia and other sites as a sanity check, and Costco is almost always cheaper. However, I was checking for an upcoming trip and found that was not the case as MB pointed out above.
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