How much is the full tank of petrol that has to be paid for in the all inclusive option for an intermediate car?

Is that the only thing that has to be paid on arrival except for child seats?

The child seat we had this year from Alamo was okay but a bit dirty and i had no help at all to fit it. Are the ones from Dollar usually a bit on the tatty (but safe) side as well? Has anyone pushed it and insisted on a better one?

I don't really want to take our own in case anything happens to it meaning we won't have one in the car to get us home.