The kilometre packages indicate the total number of kilometres that a renter can perform during the rental period. There are three different packages : 100km/day, 200km/day and unlimited package.
For instance, for a 3 day rental:
You are free to drive more kilometres in a day, as long as you do not exceed the package you have chosen. If you go over the package, extra kilometres will be paid to the owner at the price/kilometre he has set (€0,20/kilometre on average)
For 3 days of rental with the package 100/day, if you finally drive 400km (instead of the 300 initially planned), you will pay : (400-300)*0,2= €20