Once in a while I try to look up this info, but can not find it. So for the future reference, let it be known that:
In certain states your spouse is automatically an authorized driver at no extra charge.
The following wording is from National contract, however it seems that the same law applies to any car rental company.
... as required by law in California, Illinois, Indiana, Missouri, Nevada, New York, Oregon, Texas, Utah, and Wisconsin, Your spouse is an Authorized Driver if (s)he has a valid driver's license and meets the Age Restrictions...
Note: To get advantage of this rule you should not list your husband/wife on a rental agreement, otherwise the standard additional driver fee (if any) will be automatically priced in.

