Every supplier has its own policy on extra charges, and all of them are listed in the rental terms before you book. These are the ones to look for. They are normally paid at the desk, in local currency.
One-way fee
Returning the car somewhere else can add a fee, especially across borders. Many search results already include domestic one-way fees in the price, so compare the total.
Cross-border fee
Driving into another country often needs the supplier's permission, and some charge for it. Some car categories cannot leave the country at all. Tell the supplier your route when you book.
Young and senior drivers
Drivers under 25, and with some suppliers over 70, may pay a daily surcharge. The minimum age is usually 21, and some car categories require 25 or more. Enter the real driver age in the search so the price shown is accurate.
Out-of-hours pick-up or return
Collecting or dropping off outside the branch's opening hours can cost extra, if it's possible at all. Add your flight number to the booking so the desk knows if you're delayed.
Additional driver and extras
Additional drivers, child seats, GPS, winter tyres, snow chains and ski racks are usually charged per day. Child seats can sometimes be reserved in advance; availability isn't always guaranteed.
Fines, tolls and late return
Traffic fines, unpaid tolls and congestion charges are passed on to you, usually with an administration fee. Returning late can cost an extra day, so leave yourself time.
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