Thinking generically about renting cars: I often find that the normal CDW / TP cover will still come with a big excess. If you want to reduce the amount of the excess, you can take out further cover (companies may call it "Super CDW" or some such).

I wonder whether that further cover was what the €18 was for. I've never done the exact Avis / BA Holidays combination you refer to, but it wouldn't surprise me if this was what was going on.

Also, even if you do take the further cover, it won't necessarily reduce the excess to zero.