I'm not sure if Costco offers a coupon like unlimited mileage, but I do see plenty of sales and offers like 25% off Avis car rental. Costco's Low Price Finder will also automatically find the best coupon combos that work with your Costco member discount to get you the best price.
On their Rental Car Discounts page (https://www.costcotravel.com/Info/Rental-Cars-Discounts), I found this:
"Q: Does my rental waive the additional driver fee?
Yes. For Alamo and Enterprise, the additional driver fee is waived for rental in the U.S., Canada, UK, France, Germany, Ireland, Netherlands and Spain. For Avis and Budget, the fee is waived for rentals in the U.S.
The additional driver must meet all renter requirements, appear at the rental counter with the primary renter, present a valid driver's license and present a major credit card in their name at the time of rental (they do not need a valid membership card)."
Looks like additional drivers are free as long as they meet the requirements!
posted by dtran5 Jun 29, 2017