While it's too late for your deadline, I'll add my experiences here in case anyone might profit from it. I always book rental cars at the absolute rock bottom rate in the smallest sub-sub-sub Playskool sized car available. I've been known to use priceline for this purpose. On each of the last four times I've picked up my car, I've been asked if I want to upgrade to a [fill in the blank] for [fill in the dollar amount] each day. Each time I've declined, and each time I end up getting the upgraded car anyway. I've always attributed this to agent not having the sub-sub-sub compact car available, so they're trying to talk me in to paying for the upgrade that they're going to give me anyway if I decline to pay, but by trying, they at least have a shot of getting some revenue. This has happened to be with Avis, Enterprise, and National.