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


2) Create and Extra and assign this pricing scheme to it

Ecalypse rental -> Extras -> Create new item or service:


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