I don't remember anything about a deposit, but we left our credit card number anyway, so I presume this would have been used if necessary. They do try and get you to up grade to the Platinum package, that gives you an extra driver etc. but we didn't want that, so just stick to your guns. Paying for the fuel (so you return the car empty) is another one they try and get you with. This may be convenient, but if you are trying to save money they charge $2.90 something per gallon and you can buy in Miami at about $2.15 so it will cost you. We had no pressure about upgrading as that was the problem, they didn't have our SUV so we had to go for the cheaper mini van for one night.
Stick with booking the compact/economy car. They are often better than the next level up (as we found out in Washington one year) and if you don't like what you get (again often they haven't any left and they have to upgrade you for free anyway) you can always go back into the office and ask for an upgrade.