Returning early *or* extending your rental can have an adverse effect on your rate. This is especially true when you are on the border of different rate plans (i.e. daily vs. weekly, weekend vs. daily/weekly, weekly vs. monthly).

There can sometimes be an early return penalty, but other times you may get a credit. Likewise, sometimes you can just extend at the (often favorable) rate plan you're on as opposed to re-booking at a higher rate, but you need to be careful with this strategy if you're straddling the border of different rate plans as above. In short, it depends on the situation, and it takes some investigation and sometimes a bit of guesswork.

You may find this article from our blog helpful:

What Happens with Early (or Late) Car Rental Returns?