Karen: I can vouch for the fact that, as of last Fall,
AVIS charged $25 per rental (not per day) for each extra adult.
Your best option is to think about how you can get free additional drivers. For Hertz, all
AAA members can be added without a fee. I don't know if there is an equivalent for CAA.
Check with your car insurance company. With USAA, other USAA insured drivers can be added for free to Avis or Hertz.
If you are over 50, you can join the Silver Dollar Club at Dollar. Additional drivers are free, but the rates usually aren't too good otherwise.
Look at Mousesavers.com for other ideas. Sometimes there are codes at
Alamo that include one free driver.
To add just your spouse or a business associate while traveling on business, is easier. Most of the clubs, such as Emerald Club at National, or Hertz gold, include this.
Of course, rules change all the time and your actual result may vary!
Good luck in finding a good rate and the # of drivers you need!
-- Suzanne
