Car Rental Pick up Times?

[happy.family.of.6]

Hi, our car rental just informed us that 11 am in the pick up time. Partly our fault for not checking on this when we reserved it, but we really need our cars by 8 am. They said that they are full, but sometimes get cancellations, so we have to wait to call day of. (Sunday) So my question is, have any of you been able to get your car on a weekend prior to the 11 am check out time, without any problem? Like I said, we really need it by 8 am and they want to charge us double to add 1 other day?? (to get it Saturday night, instead of Sunday) Do any of you have any suggestions? Or can at least tell me that it shouldnt be a problem?

Thanks!!