What does this mean? Thrifty car rental

I'm not sure what this means. I booked a car rental through American Airlines Rewards. The car I chose was through Thrifty. The message I received after paying, stated payable at pick-up: $825. I originally agrred to what said it was 4K miles and $120. What does payable at pick-up mean? Am I gonna have to pay an additional $825?