As I understand it, you should be able to drop the car off earlier without rebooking without causing a rate change (i.e. the rate won't recalculate to that higher rate), but you will be responsible for that $15/day early return fee. Dropping off at a different location will also incur a drop fee, but that would be the same whether you did it without notification or booked in advance. So your best bet should be to leave it as is and simply return it early.

Note that if your pickup location is a franchise, you won't be able to return it to a different location (unless it's owned by the same franchise, but SEA and PDX was the last multi-city franchise that I was aware of, and it's now corporate). Franchises also don't typically charge that early-return fee, either (yet, anyway--it's still pretty new even at corporate locations). If the website lets you put in the option to drop off at your alternate location, then most likely the pickup location is corporate (and will allow the drop-off and will charge the early-return fee). Might not hurt to ask how it will work when you pick up the vehicle to prevent any surprises, though.