I found some great rates on Expedia.co.uk
Minivan for 6 days for $229- pick up at Orlando International and drop off at Miami International (compared to $527 on Dollar's website) You have to pay in advance but can cancel without penalty. (I reserved my hotel last year and this year through this site at a great rate. (had no issues last year) ASSp for $72 per night last year and $80 per night this year.)
In case anyone is interested in booking, you may want to use a cc that doesn't charge foreign transaction fees. You may also need to call your cc company so they know of the charge. Otherwise, they may decline it assuming its fraudulent.