If you have previously purchased park tickets (or plan to for this trip), you can book a rental car from
Alamo from UnderCoverTourist.com

. I have AP's for this trip so I didn't need tickets, but since I have bought from them in the past (highly recommend, BTW

), I was able to book the same car through Alamo for only $63.80 instead of the $87.62 I paid through the Alamo site

. You might want to check them out

.