In principle, you could have refused to pay it untill they tell you exactly what it is for. I would have, but I'm ornry :)
I just noticed it about a week after I rented it. I was going through the receipts and viola...a little spark went off and I started ranting.
The clerk said that no one had ever questioned him on it before and vowed to figure it out before I cam for my next tool renatl (about another week)
EBUCK