Where are you renting from?
I've seen early return penalties ($10.99/day, IIRC) at CLT and possibly other corporate locations (I'll need to go back to my receipts/rental agreements from LAX, SEA, OGG, etc. and confirm), though locations differ in policies and franchises typically have different policies than corporate stores.
I'm not sure if you can avoid the fee (if it is charged) by notifying them immediately when you pick up your car rather than keeping your original estimated return date but then returning early, but it might be worth asking. (In other words, I'm not sure if the early return fee is based on the reservation's estimated return date or the rental contract's estimated return date, since they can be different.)