When you drop the car back - especially if you're returning early - you should end your booking.
To do this:
- log in (the site will be in "trip flow")
- go to 'Step 11 - End booking'
If you can't log in, then you can reply 'X' to the SMS that you received at the start of your booking.
Remember: if you complete a booking early, the remaining time is considered “cancelled time”. You will still be charged for this time, unless another member books the car. More detail