Cheap car hire article discussion

MiddleEverything

USA - spouse as free additional driver - question 

About to book car hire for an upcoming USA holiday. Normally do this through HolidayAutos or similar to get an all-in voucher.

However, this time we are hiring in a state which doesn't say extra drivers must be free.

Some - but not all - firms offer free additional driver if they're a spouse (and we are married) - but HolidayAutos etc don't seem to have a way of saying, "I want an extra driver but don't charge me as they are my spouse and this car hire co. lets your spouse drive for free".

Ideally I'd like some certainty ahead of time rather than rock up to the desk and find out we have to pay $$$ on top.

Any words of wisdom or experience? 
I'm also trying to ask the UK broker sites and the individual US car hire firms but they're a bit slow to answer.