Pre-paying will not get you any greater probability of one vehicle over another. I'm not fond of the prepay option because if anything happens between now and then to change your trip or to alter your arrival times you could have paid in advance for a day and you might arrive late with no chance of a refund....or have a problem with a cancel if you have to cancel your trip.
The overseas booking sites do have alot of restrictions and if you don't abide by their rules and you use them to book you could have paid for something and then you don't qualify for the rental on arrival. Don't try to slip "past the rules" or you could be unhappy on arrival.

Unfortunately I don't have any suggestions for better rates. Remember the US dollar is very weak right now so the rates might show unusually high. Not sure if this is why but it's a theory.