Car hire firms offer their own Excess / Super CDW cover at the car rental station when you collect the car and sometimes through their website.
This is cover which either reduces the size of the excess you are liable for (should your rental car be stolen or involved in an accident) or removes it completely. It normally only affects the excess and does not usually extend to other elements of the insurance - so 'exclusions' such as the glass (windows,lights, mirrors etc), wheels and tyres, the undercarriage and the roof are still excluded.
Sometimes separate policies are required for Theft and Damage excesses but they can also be offered in one policy.
These Excess / Super CDW policies tend to be far more expensive (typically five to ten times more so) than independent car hire excess insurance policies available through companies represented on the MoneyMaxim car hire insurance service who all include cover for the excluded areas mentioned above.
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If you have any queries do call a member of the MoneyMaxim car hire insurance team.