If you let your clients buy extra free km for their rental, this is your suggested option
1) Create a time based pricing scheme. If you charge $20 for additional 100km per day. Make it $20
Ecalypse rental -> Pricing -> Add new pricing scheme
SAVE
2) Create and Extra and assign this pricing scheme to it
Ecalypse rental -> Extras -> Create new item or service:
SAVE
3) Assign that extra to a car of your choice
Ecalypse rental -> fleet -> modify (a vehicle) -> scroll down and check the relevant box under Extras
SAVE