icarinsurance.com. I have renewed this every year and works out at about £38 this year for any one claim up to £5000. Never had to use it (yet) and just show the hire company the document. One desk at Dublin said it's not legal, we had a differing of opinion but they still ended up not getting the excess waiver money. As above I ALWAYS video the car before and after.
I've always done the same eg Questor.
Then not falling for coercion when collecting the car, the excess insurance is more than enough.
One of their scare tactics is to take a chunky hold against your credit card.