When completing the online reservation form (make sure you are on the SXM Hertz site, not the USA site), there's a drop-down for you to pick the size car you want. At the top of the drop-down is their most expensive model so you need to be careful with that. I just made a reservation for next June and received an automated acknowledgement almost immediately. The quote was received the next day. I will say I am a little disappointed with them as, even this far out, they don't have any "Class B" (economy) cars available next June. They quoted me a sedan, which isn't my preference. I also have a sedan for this coming November's trip even though I made that reservation many months ago. I think they are phasing out the economy cars due to the high theft rate but don't really know that for a fact.

LINK Hertz Online Reservation Form