Other Car Rental Programs/Partners (ie. Alamo, Enterprise, Sixt) - Strange rate difference from Alamo

I was booking a one-way rental of 4 days, 6 hours from Miami to Key West in December. An off-airport Alamo location has a rate of $43/day all-in for an economy car for a rental of exactly four days, but adding even one extra hour to the four days increases the daily rate to $92/day for the same car from the same location.

I think I'm going to book it for exactly four days. They can't possibly charge me $200+ just for keeping the car an extra few hours, can they? Even if so, I'll return it early and get a new car for the last day.

I'm assuming the 4 days is between Friday to Monday. That's the weekend rate which is always much cheaper. Adding in a day (Thursday or Tuesday) can nullify the weekend rate and make it a regular daily rate.

Sucks, I know.

It's Sunday-Thursday.